Principal deputy U.S. national security adviser Jon Finer visited Equatorial Guinea in October on a mission to persuade President Teodoro Obiang Nguema Mbasogo and his son and heir apparent, Vice President Teodoro “Teodorin” Nguema Obiang Mangue, to reject China’s overtures.

“As part of our diplomacy to address maritime-security issues, we have made clear to Equatorial Guinea that certain potential steps involving [Chinese] activity there would raise national-security concerns,” said a senior Biden administration official.

The great-power skirmishing over a country that rarely draws outside attention reflects the rising tensions between Washington and Beijing. The two countries are sparring over the status of Taiwan, China’s testing of a hypersonic missile, the origins of the Covid-19 pandemic and other issues.

World-wide, the U.S. finds itself maneuvering to try to block China from projecting its military power from new overseas bases, from Cambodia to the United Arab Emirates.

In Equatorial Guinea, the Chinese likely have an eye on Bata, according to a U.S. official. Bata already has a Chinese-built deep-water commercial port on the Gulf of Guinea, and excellent highways link the city to Gabon and the interior of Central Africa.

The “most significant threat” from China would be “a militarily useful naval facility on the Atlantic coast of Africa,” Gen. Stephen Townsend, commander of U.S. Africa Command, testified in the Senate in April. “By militarily useful I mean something more than a place that they can make port calls and get gas and groceries. I’m talking about a port where they can rearm with munitions and repair naval vessels.”

Equatorial Guinea, a former Spanish colony with a population of 1.4 million, secured independence in 1968. The capital, Malabo, is on the island of Bioko, while Bata is the largest city on the mainland section of the country, which is wedged between Gabon and Cameroon.

Mr. Obiang has ruled the country since 1979. The discovery of huge offshore gas and oil reserves in 1996 allegedly allowed members of his family to spend lavishly on exotic cars, mansions and other luxuries, according to U.S. Senate and Justice Department investigations.

Contacted by The Wall Street Journal, Gabriel Mbaga Obiang Lima, Equatorial Guinea’s oil minister and one of the president’s sons, requested that questions about his country’s relationship with China and allegations of corruption in his family be submitted in writing. He didn’t respond to those questions. Equatorial Guinea’s ambassador in Washington didn’t respond to multiple interview requests.

U.S. intelligence agencies began picking up indications of China’s military intentions in Equatorial Guinea in 2019. During the closing days of the Trump administration, a senior Pentagon official visited the country, but the approach apparently left the Obiangs uncertain about how seriously the U.S. took China’s military aspirations.

The Biden White House has sought to deliver a sharper message: It would be shortsighted of Equatorial Guinea to insert itself between the front lines of U.S.-China global competition.

At the same time, the U.S. has taken steps to warm relations. In March, the U.S. offered aid after an apparently accidental ammunition explosion leveled an army base near Bata, killing at least 100 people.

The same month, Equatorial Guinean troops participated in U.S.-led naval exercises in the Gulf of Guinea. In August, an American Navy ship anchored off the Bata port, and the captain invited local officials and naval personnel aboard to observe firefighter training.

The White House doesn’t know whether its diplomatic outreach will have the desired effect and believes it will require a persistent, long-term effort to fend off a Chinese naval presence.

At the same time, the U.S. wants to convey a nuanced message: Washington isn’t asking Equatorial Guinea to abandon its extensive ties with China, but just to keep relations within bounds the U.S. considers unthreatening.

The U.S. concern “is that the Chinese would develop a naval base in Equatorial Guinea, which would then give them naval presence on the Atlantic,” Maj. Gen. Andrew Rohling, commander of the U.S. Army Southern European Task Force—Africa, said in a June interview.

Following the visit by Mr. Finer, Mr. Obiang Mangue, the president’s son and de facto head of Equatorial Guinea’s security forces, announced that the White House had named him “the No. 1 interlocutor in relations between our two countries.”

He tweeted a thank-you video showing the protocol gift he received from Mr. Finer’s delegation, a silver platter engraved with the U.S. presidential seal. A few days later, Mr. Obiang Mangue and the chargé d’affaires at the U.S. Embassy in Malabo discussed proposals raised during Mr. Finer’s visit.

Shortly afterward, however, Mr. Obiang, the president, spoke by phone with Chinese President Xi Jinping, after which Beijing put out a statement highlighting that “Equatorial Guinea has always regarded China as its most important strategic partner.”

China helps train and arm the Equatorial Guinean police.

The Chinese Ministry of Foreign Affairs didn’t respond to a written request for comment on any basing plans in Equatorial Guinea or elsewhere on Africa’s Atlantic coast.

Beijing set up its first overseas military base in 2017 in Djibouti, on the opposite side of the continent. The former French colony looks onto the Bab-el-Mandeb strait, a strategic chokepoint for shipping traffic transiting the Suez Canal. The Chinese facility has a pier capable of docking an aircraft carrier and nuclear submarines, according to U.S. Africa Command.

The base is 6 miles from the largest American base in Africa, Camp Lemonnier, home to 4,500 U.S. troops.

“China doesn’t just build a military base like the U.S.,” said Paul Nantulya, research associate at the Pentagon-funded Africa Center for Strategic Studies. “The Chinese model is very, very different. It combines civilian as well as security elements.”

Chinese state-owned companies have built 100 commercial ports around Africa in the past two decades, according to Chinese government data.

This spring, U.S. intelligence officials uncovered what they said was construction on a secret military base at a Chinese-run commercial port in the United Arab Emirates. The Biden administration persuaded Emirati authorities to halt construction, at least temporarily.

American diplomats in Mauritania, along Africa’s northwest coast, have advised local authorities to rebuff any effort by Beijing to use a Chinese-built port for military purposes, according to a U.S. official.

In a report to Congress this year, the Pentagon said China “has likely considered” African bases in Kenya, Seychelles, Tanzania and Angola.

There are no visible signs of major construction at the Bata port, which was upgraded by China Road & Bridge Co., a state-owned enterprise, between 2009 and 2014.

The U.S. knows it faces challenges in its bid for Equatorial Guinea’s favor, seeking help from a country it has pointedly criticized.

The State Department has accused the Obiang regime of extrajudicial killings, forced disappearances, torture and other abuses.

A U.S. Senate committee issued a report in 2004 criticizing Washington-based Riggs Bank for turning “a blind eye to evidence suggesting the bank was handling the proceeds of foreign corruption” in accepting hundreds of millions of dollars in deposits controlled by Mr. Obiang, his wife and other relatives.

The bank said it regretted that it “did not more swiftly and more thoroughly complete the work necessary to fully meet the expectations of our regulators.” PNC Financial Services Group Inc. acquired Riggs the following year.

Separately, the U.S. Justice Department pursued the allegedly ill-gotten gains of the president’s son Mr. Obiang Mangue.

In 2011, Mr. Obiang Mangue called the U.S. ambassador in Malabo asking for help clearing his name against what he said were unfair allegations surfacing in the press. “I have never stolen money from our country’s treasury,” he told the ambassador, according to a State Department cable entered into court records. He told the ambassador he had earned his riches by winning legitimate government contracts during the country’s oil-fueled infrastructure boom.

In a series of civil cases, however, U.S. government lawyers accused Mr. Obiang Mangue of amassing a fortune of more than $300 million “through corruption and money laundering” while earning less than $100,000 a year as minister of agriculture and forestry. In a 2014 settlement, Mr. Obiang Mangue surrendered to the federal government proceeds from a Malibu mansion, a Ferrari and other assets.

This fall, the Justice Department announced that it would steer $26.6 million of the surrendered assets back to Equatorial Guinea in the form of Covid-19 vaccines and other medical aid, bypassing the government.

The Equatorial Guinean Foreign Ministry responded with a statement condemning the U.S. announcement as a “misrepresentation” of the facts. In a series of tweets, Mr. Obiang Mangue said it had been his desire to use the funds for medicine and that the U.S. government hadn’t forced him to do so.

Though often at odds with the Obiang regime, the U.S. isn’t without leverage.

Equatorial Guinea relies on American oil companies to extract offshore resources that have made the country the richest on the sub-Saharan mainland, as measured by per capita annual gross domestic product.

And the State Department recently raised Equatorial Guinea’s ranking in the annual assessment of how diligently countries combat human trafficking. The upgrade could allow the Biden administration to offer maritime-security assistance to help win Equatorial Guinea’s cooperation.

The country faces a growing threat from pirates and illegal fishing in its waters on the Gulf of Guinea.

“We think there is a fair amount we could do together on the maritime-security side that would be in our interest and of interest to them,” said the senior administration official.