At least four people were confirmed killed and 20 seriously injured in a fresh strong aftershock in southwestern China, officials said on Monday.

The aftershock measuring 6.4 on the Richter scale hit Qingchuan county in southwestern Sichuan province on Sunday afternight, causing the loss of lives and properties.

All the dead were from Hanzhong city in Shaanxi province.

Hu Runze, mayor of Hangzhong, said all the four were in a paddy field when the aftershock occurred.

Investigation has started for the detailed losses caused by the aftershock, Hu said.

On May 12, 2008 the Sichuan province was struck by a 8.0-magnitude quake that killed more than 62,000 people and injured another 350,000.

Some 24,000 people were still missing. A total of 638,305 people had been rescued and resettled to safer places by the military and civilian rescuers, said Guo Weimin, Director of the information office on Sunday.

According to authorities, some 24,000 people are still missing.

The Sunday's aftershock was the strongest of the 8,000 tremors that rocked Sichuan province in the aftermath of the May 12 quake.