BEIJING – China has launched a probe into the high number of school collapses in Monday's earthquake that killed thousands of children and buried thousands more when many other buildings remained standing.

The 7.9 magnitude quake struck in the middle of the afternoon in the southwest province of Sichuan when most children were at their desks or taking a nap.

A score of high-profile cases of hundreds of children being killed instantly or buried beneath their schools has fuelled anger from parents who have accused authorities of cutting costs and failing to meet safety standards.

China put the quake's total death toll at around 22,000 on Friday, including 21,577 in Sichuan, but has said it expects the toll to exceed 50,000.

'I am very saddened by the deaths of students. If investigations show that is the case, it must be dealt with seriously,' Jiang Weixin, Minister of Housing and Urban-Rural Construction, told reporters at news conference carried live on state television.

Bloggers and state media have also raised questions after pictures of collapsed schools surrounded by relatively unscathed buildings.

Jiang side-stepped a question on whether he had noticed a disproportionate amount of schools had collapsed, saying that the toppled schools he had visited were surrounded by other ruins.

But he conceded that corner-cutting may have played a part.

'At this stage we also cannot rule out the possibility that there may have been shoddy work and inferior materials during the construction of some school buildings,' Jiang said.

He said that buildings meeting local standards were simply not strong enough to withstand the power of the 7.9 magnitude quake.

'Buildings in Sichuan have been built to withstand earthquakes that have an intensity of up to seven, but the Wenchuan tremor had an intensity of 10-11,' Jiang said.

The Mercalli scale quantifies the intensity of earthquakes ranging from one to 12.

'The overwhelming majority of buildings, particularly those near the epicentre, would collapse with such an intensity,' Jiang said.

Jiang said more than 4 million houses had collapsed or were damaged in Sichuan province, with another 700,000 'affected' in neighbouring Gansu and Shaanxi provinces.

Tap water systems in about 20 cities and counties in Sichuan had also been seriously damaged, along with power grids and natural gas utilities.

Jiang said his ministry had been mobilising water purifiers, gas cookers and mobile toilets to be shipped to Sichuan, but warned that Sichuan's 1.2 million migrant workers in other provinces would not be encouraged to return to aid in the stricken region's recovery.

'The transportation is not convenient and it may add new pressure to relief work. They'd better not return to Sichuan but just stay where at their work places,' Jiang said.

(Reporting by Ian Ransom and Guo Shipeng; Editing by Nick Macfie and Valerie Lee)