China struggled to bury its dead and help tens of thousands of injured and homeless yesterday when a powerful aftershock brought new havoc four days after an earthquake thought to have killed more than 50,000.

President Hu Jintao flew to the battered province of Sichuan and Premier Wen Jiabao said the quake damage could exceed the devastating 1976 tremor in the northeastern city of Tangshan that killed up to 300,000 people.

Mr Wen called on officials to ensure social stability as frustration and exhaustion grew among survivors, many of whom lost everything and were living in tents or in the open air.

China put the death toll at just over 22,000 yesterday but has said it expects it to exceed 50,000. About 4.8 million people have lost their homes.

Thousands of men, women and children were heading on foot for Mianyang, a city near the epicentre, saying they were abandoning their ruined villages for good.

Anger has focused on the state of school buildings, many of which crumpled in Monday's 7.9 magnitude quake, burying thousands of children and prompting the Housing Ministry to order an investigation.

Mr Hu and Mr Wen stressed that searching for and rescuing survivors remained the top priority.

"We cannot talk about giving up too easily," Mr Wen said. "Life should go on. I believe people in the quake area can definitely build their hometown even better with their own hands. That is also the biggest consolation for the dead."

The country is on precautionary alert against possible radiation leaks, according to a government website seen yesterday.

The disaster area is home to China's chief nuclear weapons research lab in Mianyang, as well as several secretive atomic sites, but no nuclear power stations.