He said 50 sources of radiation were buried by debris from the massive 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 sources was far higher than the two the government reported earlier this week. Foreign observers said the radioactive sources probably came from materials used in hospitals, factories, or in research, not for weapons.
US 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 United States was relying for much of its information on the Chinese government.
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 numerous 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 quake-hit area were forced to stop production as a result of damage, he said.
No environmental damage has so far been recorded, but as factories begin production, officials would need to vigilant to ensure chemical waste found of produced was 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.
