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."