CHENGDU, China — A stockpile of chemicals being used to disinfect an earthquake-shattered Chinese town ignited yesterday and injured scores of soldiers doing relief work, adding to a day of problems for urgent recovery efforts.

Heavy rain also added to the misery of crowds of homeless survivors living in tents or lean-tos, and hampered troops rushing to drain a quake-spawned lake before it floods a valley filled with villages.

The chemical fire took place in the town of Leigu, in devastated Beichuan county. The official Xinhua News Agency reported that more than 800 people were evacuated to avoid a cloud of dense chlorine gas caused by the blaze.

As in many destroyed towns, officials have been spraying disinfecting bleach on streets and rubble in an effort to prevent disease breakouts. Thousands of people are still missing and their bodies could be buried in the rubble, while rats and other scavengers have been reported in some places.

State-run television showed smoke billowing over Leigu and reported that a stockpile of bleach powder had ignited in a storage building. CCTV footage showed soldiers spraying down the building and extinguishing the threat, and then several soldiers who were gasping for air being treated by medics.

“The soldiers have inhaled the fumes, it has affected their bodies, and they are in the military hospital now,” a soldier, identified by CCTV as the leader of the fire crew, said. His name was not given. He said 61 soldiers were injured. Xinhua reported that four people were injured. It was not immediately possible to reconcile the different injury tolls.

It was not immediately clear why the bleach ignited, though substances in it can turn explosive if heated or mixed with hydrocarbons such as those in diesel fuel.