Vincent Yu, Associated Press
Residents of hard-hit Beichuan County evacuate the area Saturday. The man in the wedding photo was killed in Monday's huge quake.

CHENGDU, China — Thousands of earthquake survivors fled tent camps and villages across the ravaged landscape of southwestern China on Saturday after the government warned that several lakes and rivers were getting dangerously close to overflowing because landslides have blocked water flow.

The new threats came as government officials said that more than 3 million homes were destroyed by last Monday's earthquake and more than 12 million were damaged. The government also once again increased the death toll, to nearly 29,000. The resulting humanitarian crisis is the largest China has faced in decades.

With the scale of the disaster becoming ever more apparent, the United Nations announced that it would provide a grant of $7 million from an emergency response fund "to help meet the most urgent humanitarian requirements."

The danger of flooding was so severe that some rescue workers had to abandon their efforts, at least temporarily, to find people buried beneath rubble in Beichuan, one of the hardest-hit counties. With the chances of finding survivors dwindling by the hour so long after the quake, such interruptions could doom the relatively few who could be expected to be alive beneath debris.

The greatest danger of flooding came from a lake in the far north of Sichuan province that had already begun to overflow because of a blockage in the Qingzhu River, according to the Xinhua news agency, citing experts at the province's land management department. Heavy rains began pounding large swaths of Sichuan on Saturday night, adding to the threat.

A rise of only 6 feet to 10 feet will cause the lake to "threaten more than 2,000 people who are staying in shelters after the earthquake downstream," said one expert. The expert added it was inevitable that debris would continue to flow down, adding to the blockage.

Early today, a tremor with a magnitude of 6.0 struck northern Sichuan, one of the largest quakes since last Monday; other tremors over the past several days have caused new landslides.

Relief officials in the county where the flood threat is highest, Qingchuan, have begun evacuating people and are considering blasting the embankment to divert water from the overflowing lake.

"We were informed that the Qingchuan government is requesting urgent evacuation because the water level of the dammed lake has reached 70 meters," said a worker at the control center of the Guangyuan Petrol Co., who gave his name as Wang. "We're evacuating all our staff working at gas stations in Qingchuan right now."

Farther south, closer to the epicenter, people around the county seat of Beichuan, which was flattened by the 7.9-magnitude earthquake, also began fleeing because of flood warnings related to a choked river.

Thousands of people are buried beneath rubble in Beichuan, and it is the scene of one of the most intense rescue efforts in the disaster zone. But on Saturday, soldiers, rescue workers and medics had to stop their work and seek higher ground or leave the area. People who were fleeing described soldiers from the People's Liberation Army heeding the flood warning, packing up and driving down the main road leading from the mountains.

There were unconfirmed reports that the immediate danger had passed by evening and that rescue operations had resumed.

Landslides continue to pose one of the greatest threats across the rugged mountainous terrain of Sichuan. Daily aftershocks and tremors — at least 168 significant ones since Monday — set off new slides, further damaging already fragile settlements and blocking crucial roads. There are at least 13 rivers and lakes that have been dammed up by the quake, the state-run China National Radio reported Saturday, citing an official at the Land and Resource Ministry.

On Wednesday, the top economic planning body in China issued a report saying that 391 reservoirs had suffered damage from the earthquake, two of them large and 28 mid-size. The same day, 2,000 soldiers were sent to inspect cracks in the Zipingpu dam, upriver from the hard-hit town of Dujiangyan, and drain water from the reservoir.

Experts outside China say many of the threatened dams and reservoirs were built along the well-recognized Longmen Shan fault and that the dams might have sustained damage that could cause them to fail weeks later.

Government officials said Saturday that the death toll had risen to 28,881. Earlier in the week, they said the toll could rise as high as 50,000. Officials said Saturday that 12.5 million homes had been damaged and 3.1 million had collapsed.

President Hu Jintao has urged rescuers to continue searching for survivors. Some were pulled out on Saturday, but medical experts say the chances of people living in rubble decrease significantly after the first 72 hours.

The Ministry of Health said Saturday that it had found no epidemics in the disaster areas, Xinhua reported. Hospitals in Sichuan had received more than 116,000 patients, 14 percent of whom were severely injured, the ministry added. More than 34,000 medical workers and disease control staff are in Sichuan Province, and they are being given pamphlets that tell them how to disinfect food and drinking water and how to handle corpses.

Contributing: Zhang Jing, New York Times News Service