BEIJING — Twenty-seven people died en route to a COVID-19 quarantine facility when their bus crashed in southwest China on Sunday, local authorities said, in the country's deadliest road accident this year.

The accident took place on a highway in rural Guizhou province when the vehicle carrying 47 people “flipped onto its side”, Sandu county police said in a statement.

Emergency responders were dispatched to the scene and another 20 people were being treated for injuries. The accident happened in Qiannan prefecture — a poor, remote and mountainous part of Guizhou, home to several ethnic minorities.

The Guizhou government confirmed later Sunday that the vehicle had been "transporting people linked to the epidemic to quarantine" from the provincial capital of Guiyang, and that the accident occurred around 2:40 a.m. (1840 GMT).

"At present, on-site rescue work is basically completed, the treatment of the injured and aftercare of the deceased are being carried out in an orderly manner, and the cause of the accident is under investigation," the local government said in a social media statement.

It was not clear whether the passengers were infected with COVID, close contacts, or living in the same building as virus patients.

Social media users angrily demanded to know why a passenger bus was traveling down a highway in the early hours when many major roads in the province have been closed to regular traffic.

One hundred toll stations are shuttered in Guizhou because of COVID-19 restrictions and long-distance passenger journeys across China are banned from running between 2 a.m. and 5 a.m.

Guizhou is in the middle of a COVID outbreak that has seen more than 900 new infections in the past two days alone. Its provincial capital, Guiyang, home to six million residents, was locked down earlier in September.

The bus was traveling south in the direction of Guiyang to Libo county, according to the police statement. Road accidents remain common in China where irregular enforcement and lax safety standards have resulted in many fatalities over the years.

Photos shared widely on social media Sunday showed a gold-colored passenger bus, its top is completely crumpled, being towed by a truck.

Another viral photo appeared to show the bus driving at night, with the driver and passengers wearing hazmat suits, which are still commonly worn in China to protect against COVID.

"This feeling can't simply be represented by lighting a candle and saying RIP," read one Weibo post with more than 15,000 likes.

Some people on social media used the accident to criticize China's unrelenting zero-COVID policy, which has often seen entire housing compounds of thousands relocated to purpose-built quarantine facilities, sometimes hundreds of kilometers away.

Guizhou's Communist Party chief and the provincial governor "rushed" to Qiannan prefecture to direct emergency response work, the local government said, adding the officials "expressed deep condolences to the victims".

"It is necessary to draw a lesson from the accident, examine the quarantine and transportation of epidemic-linked personnel and hidden dangers in traffic safety... resolutely curb the occurrence of major accidents," the statement said.

Guizhou officials also vowed to set up a working group to investigate the cause of the accident.

Guizhou has also seen other transport accidents. In June, a railway driver was killed when a high-speed train derailed in the province.

And in March, a Chinese passenger jet crash killed all 132 people on board, marking the deadliest aviation accident to take place in China for decades. — Agencies