China’s official death toll from the new coronavirus spiked dramatically on Thursday after authorities changed their counting methods, fuelling concern the epidemic is far worse than being reported.

As the figures soared in China, a troubling new front opened abroad as neighbouring Vietnam placed 10,000 people under quarantine after six COVID-19 cases were discovered in a cluster of villages — the first such lockdown overseas.

Japan reported its first death of an infected person — the third fatality outside mainland China after the Philippines and Hong Kong.

Under criticism at home over the handling of the crisis, China’s Communist Party sacked two top-ranking officials in Hubei province, the epicentre of the outbreak.

The developments came hours after President Xi Jinping claimed “positive results” from efforts to contain an epidemic that has now officially killed 1,367 people and infected nearly 60,000.

In Hubei and its capital Wuhan, where tens of millions of people are trapped as part of an unprecedented quarantine effort, 242 new deaths were reported on Thursday.

Another 14,840 people were confirmed to be infected in Hubei alone, with the new cases and deaths by far the biggest one-day increases since the crisis began.

But the World Health Organization moved to calm fears, saying the new numbers did “not represent a significant change in the trajectory of the outbreak.”

“This increase that you’ve all seen in the last 24 hours is largely, in part, down to a change in how the cases are being reported,” Michael Ryan, head of WHO’s health emergencies programme, told reporters in Geneva.

Outside Hubei, there were 12 more deaths but the number of new cases fell for a ninth day in a row, with 312 extra patients.

Hubei authorities said the increases were because they had broadened their definition for infection to include people “clinically diagnosed” via lung imaging.

Up until now, they had solely confirmed cases with a more sophisticated laboratory test.