TOKYO — Amid widening alarm in the United States and elsewhere about Japan’s nuclear crisis, military fire trucks began spraying cooling water on spent fuel rods at the country’s stricken nuclear power station late Thursday after earlier efforts to cool the rods failed, Japanese officials said.

The United States’ top nuclear official followed up his bleak appraisal of the grave situation at the plant the day before with a caution that it would “take some time, possibly weeks,” to resolve.

The developments came as the authorities reached for ever more desperate and unconventional methods to cool damaged reactors, deploying helicopters and water cannons in a race to prevent perilous overheating in the spent rods of the No. 3 reactor.

Moments before the military trucks began spraying, police officers in water cannon trucks were forced back by high levels of radiation in the same area. The police had been trying to get within 50 yards of the reactor, one of six at the plant.