PENGZHOU, China — Emergency crews worked Friday to secure 15 sources of radiation buried in the rubble of China’s devastating earthquake, the government said as it evacuated thousands of survivors downstream from rivers dammed by landslides.

Officials precariously balanced their efforts to clean up and rebuild with attempts to house, feed and treat the displaced and injured and search for survivors.

One senior official said China faces “a daunting challenge” to prevent environmental contamination from other sources.

There has been no leak of radioactive substances into the environment, Wu Xiaoqing, China’s vice minister for environmental protection, told reporters in Beijing.

He said 50 sources of radiation were buried by debris from the May 12 earthquake in central China, 35 of which had been secured. The rest lay buried or located but unreachable under collapsed buildings. He gave no specifics about the radiation sources.

The number of unsecured radiation sources was far higher than the two the government reported earlier this week. Foreign experts say the radioactive sources likely came from materials used in hospitals, factories or in research, not for weapons.

U.S. State Department spokesman Tom Casey told reporters that Washington was not aware of any threat to humans, “but, obviously, it is a concern.”

He added that many of the locations were remote and the U.S. was relying on the Chinese government for much of its information.

Wu cautioned that a number of other “hidden” sources of pollution are likely to be encountered as workers begin digging into the rubble, which includes many factories and refineries.

The worst-hit areas in Sichuan province include many high-risk petrochemical and chemical companies, he said. Around three-fourths of the more than 100 chemical plants in the disaster zone were forced to stop production because of damage, he said.

No environmental damage has been recorded, but as factories begin production, officials would need to be vigilant to ensure chemical waste is properly disposed of, he said.

“We now face more daunting challenges in investigating and removing hidden troubles and carrying out environmental inspections as time passes by and factories gradually resume production,” he said.

As the government continued to bring relief to the devastated areas of Sichuan, it was evacuating thousands downstream from rivers that were blocked by landslides.

With their waters pooling, the rivers could breach the earthen barriers, a danger that would grow with coming rains or because of aftershocks.

The confirmed death toll rose to 55,740 and 24,960 others remained missing, said the State Council, China’s Cabinet.

The government was shifting focus to reconstruction and away from the search for survivors and bodies among the wreckage.

Chinese banks were told Friday to forgive debts owed by survivors in an effort to revive the economy, and the government warned it was cracking down on price-gouging by merchants in the disaster area.

The government also said it sent more fuel from China’s strategic reserves to quake-affected areas.

Six pandas were moved Friday from a damaged panda-breeding base in Wolong near the epicenter because of food shortages, the official Xinhua News Agency said. They were taken to another preserve in Sichuan, near a city called Ya’an.

Two pandas missing since the quake have still not been located.

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Associated Press writer Cara Anna, Joe McDonald and Henry Sanderson in Beijing contributed to this report.