Japan's chief cabinet secretary dismissed reports that Japanese officials had rejected previous offers of U.S. assistance and said Japan is "coordinating with the U.S. government as to what the U.S. can provide and what people really need.
"We have repeatedly asked for specific support, and indeed, they are responding to that," he added.
A nine-member assessment team will determine whether it makes sense to bring in a larger American force to Japan. That larger force would be the CCMRF team that ABC News' Martha Raddatz has reported is being considered for deployment to Japan.
Admiral Willard, the head of U.S. Pacific Command, has requested "a force of about 450 radiological and consequence-management experts to be available to us," he said, referring to the CCMRF. "They're on a prepare-to-deploy order. And one of the purposes of that advance team is to assess the need to call them forward."
Those forces would carry radiation detection equipment as a precaution and operate within a recommended U.S. evacuation radius, as needed.
The Japanese government maintains a mandatory evacuation order for a 12-mile radius around Fukushima Daiichi. Residents within 20 miles are told to stay indoors with windows sealed.
The United States has suggested a 50-mile evacuation radius and has offered chartered flights to take American military dependents who want to leave out of the area.
The estimated number of Americans in Japan range from 160,000 to 350,000 at any given time -- though such numbers are only guesses because Americans are not required to register with the U.S. embassy in Japan, the State Department said.
The U.S. embassy in Tokyo believed there were about 1,300 Americans in the tsunami-affected area.
The first charter flights left Japan Friday morning for authorized voluntary departures of military dependents, but the number of evacuees was relatively small.
ABC News' Akiko Fujita, Kirit Radia, Luis Martinez, Ann Compton, Michael S. James and James Hill, and The Associated Press contributed to the story.