"We have consular teams on the ground," Toner said. "Where they can, they are going door to door. They are going to hospitals. They are trying everything in their power to reach out and find American citizens."
The Pentagon said U.S. troops working on relief missions can get closer than 50 miles to the plant with approval. Lapan said the U.S. would review requests from the Japanese for assistance that would require troops to move within that radius, though no approval for such movement had been given since the stricter guidelines were enacted.
The Pentagon said troops are receiving anti-radiation pills before missions to areas where radiation exposure is likely.
"U.S. forces remain in Japan and the U.S. has full capability to fulfill our alliance commitments to defend Japan and maintain peace and security in the region," Lapan said.
With the arrival of three more ships to the massive humanitarian mission, there were 17,000 sailors and Marines afloat on 14 vessels in waters off Japan. Several thousand Army and Air Force service members already stationed at U.S. bases in Japan have also been mobilized for the relief efforts.
Airmen have been flying search and rescue missions and operating Global Hawk drones and U-2 reconnaissance planes to help the Japanese assess damage from the disasters. The operation is fraught with challenges — mainly, figuring out how to continue to provide help amid some low-level releases of radiation from the facility, which officials fear could be facing a meltdown.
Weather also temporarily hampered some relief plans Wednesday. Pilots couldn't fly helicopters off the deck of aircraft carrier USS Ronald Reagan until late afternoon because of poor visibility. The 7th Fleet said 15 flights with relief supplies were launched from the eight-ship carrier group, about half as many as the 29 flights reported the previous day to deliver food, water, blankets and other supplies.
Several water pumps and hoses were being sent from U.S. bases around Japan to help at Fukushima, where technicians were dousing the overheating nuclear reactors with seawater in a frantic effort to cool them. The U.S. had already sent two fire trucks to the area to be operated by Japanese firefighters, said Cmdr. Leslie Hull-Ryde, a Pentagon spokeswoman.