Washington (CNN) -- The State Department announced late Wednesday that it has approved the departure of family members of U.S. government personnel from certain areas of Japan in the aftermath of the earthquake, the tsunami and the nuclear power plant crisis.
Charter flights will be made available to the approximately 600 people, according to Under Secretary of State Patrick Kennedy.
"When we do a voluntary authorized departure, the State Department bears the expense of the transportation," Kennedy said.
"There are still commercial seats available out of Tokyo," he said. "However, because we do not wish to consume large numbers of seats that others might need, we are making arrangements to bring a couple of chartered aircraft into Tokyo for both the official U.S. government family members who have chosen to leave and for any American citizens who might need assistance."
Americans are urged to postpone all travel to the affected areas, the U.S. said, hours after expanding the recommended evacuation zone around the nuclear plants to a radius of 50-miles.
The Japanese government has told people to evacuate to at least 20 kilometers (12 miles) away from the damaged reactors.
"The State Department has authorized the voluntary departure from Japan of eligible family members of U.S. government personnel assigned to the U.S. Embassy in Tokyo, the U.S. Consulate in Nagoya, and the Foreign Service Institute Field School in Yokohama," the announcement said.
"U.S. citizens should defer all travel to the evacuation zone around Fukushima Daiichi Nuclear Power Plant, areas affected by the earthquake and tsunami, and tourism and nonessential travel to the rest of Japan at this time," the announcement said.
The Pentagon announced late Wednesday that it also would assist in the departure of dependents of its military and Defense Department personnel from crisis areas in Japan.
The U.S. military has about 43,000 family members in Japan, but it is unclear how many are in the affected areas, said Col. Dave Lapan, a Pentagon spokesman.
CNN's Larry Shaughnessy contributed to this report.