DUJIANGYAN, China (Reuters) - China ordered fresh waves of helicopters and aid to earthquake-devastated areas as the sheer magnitude of caring for tens of thousands of homeless survivors threatened to overwhelm relief efforts.

The Communist Party leadership told officials to "ensure social stability" as Monday's 7.9 magnitude quake in southwest Sichuan province spawned rumors of chemical spills, fears that dams could burst and torrid scenes of collective grief.

The official death toll estimate from the Monday quake stood at 14,463. But as search teams sift, often bare-handed, through towns turned to rubble, that number is likely to balloon. And the strains from tens of thousands of homeless are growing.

In Dujiangyan and other hard-hit cities and towns, many residents spent the night sleeping outside or in makeshift shelters, fearing more tremors and building collapses.

"In one minute the city we know flew away. I never dreamt it could happen," said He Lixia, a teacher in Dujiangyan, where many people slept outdoors.

Rescuers there wrapped corpses dragged from the rubble in tarpaulins, and after allowing relatives to briefly mourn, sped them to morgues. They were so busy that a notice outside one collapsed school, where locals said nearly 300 children died, asked parents to go to search for missing children in shifts.

"Because there is a heavy work load at the morgue we have arranged first for the parents of years 1, 2, 3 students to go and then organize for the parents of year 4, 5, 6 students," it said.

Wang Yujie, a teacher whose school withstood the quake, said getting more water was now residents' most immediate need.

"There is enough food but not enough water, we have only had bottled mineral water the past few days, nothing to cook with," she said as she queued beside a water truck.

By Thursday, unhygienic waste was also a growing problem as temporary toilets failed to keep up with demand.

In Shifang, another small Sichuan city that covers many villages, 30,000 of some 430,000 residents were missing or out of contact, local officials told Xinhua.

The ruling Chinese Communist Party's Standing Committee met late on Wednesday to assess the calamity that has thrown a dark shadow over preparations for the Beijing Olympics in August.

The government ordered fresh waves of troops to supplement the tens of thousands already dispatched, and more than 100 more helicopters to help send rescuers and supplies to areas blocked by buckled roads, state media said.

"As long as there is a glimmer of hope, spare no efforts in rescuing," the Party leadership ordered, according to Xinhua. "Ensure social stability in the disaster zone."

But the waves of rescuers appear to be hampered by lack of specialized equipment.

Television has shown droves off them straining to hand-lift concrete blocks, and into the fourth day since the quake, hopes of pulling survivors from crumpled schools, homes and factories are dimming.

DANGEROUS DAMAGE

The devastated 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.

The China Nuclear Engineering and Construction Corp reported that several of its facilities in Sichuan were damaged by the quake, with six staff killed. One project was stopped and workers were evacuated, it said.

The report on the corporation's Web site (www.cnecc.com) did not describe the facilities or mention any radiation leaks. A Western expert knowledgeable about the Mianyang lab said it was not likely that those facilities were put at serious risk. He requested anonymity.

Officials have also warned of dangers from increased strain on local dams as well as mudslides on brittle hillsides where rain has been forecast over the next few days.

Landslides had blocked the flow of two rivers in northern Qingchuan county, forming a huge lake in a region where 1,000 have already died and 700 are buried, Xinhua said.

"The rising water could cause the mountains to collapse. We desperately need geological experts to carry out tests and fix a rescue plan," Xinhua quoted Li Hao, the county's Communist Party chief, as saying.

Premier Wen Jiabao, a geologist himself, has criss-crossed the disaster zone and made emotional appeals to workers and comfort orphaned children.

"To be responsible to the people, be responsible to history, we must do rescue organization work at this crucial moment," he said late on Wednesday, according to Xinhua.

The quake was the worst to hit China since 1976 when up to 300,000 died. Leading disaster modeling firm AIR Worldwide said the cost of the quake was likely to exceed $20 billion.