President Barack Obama held a news conference today to reassure Americans that the United States is not at risk to harmful levels of radiation exposure stemming from the nuclear plant damage in Japan.

The president said he does not expect Americans to take any precautionary measures other than staying informed.

Speaking from the Rose Garden outside the White House, Obama said the United States has begun to evacuate Americans from the 50-mile radius nearest the Fukushima nuclear plant. The decision, he said, was made following “careful scientific evaluation.”

"I know that many Americans are also worried about the potential risks to the United States. So I want to be very clear. We do not expect harmful levels of radiation to reach the United States, whether it's the West Coast, Hawaii, Alaska, or U.S. territories in the Pacific," said Obama.

There have been discrepancies between the American and Japanese assessments of the severity of the nuclear plant damage and hazard to the environment.

Obama said he also authorized the volunteer departure of Americans in northeastern Japan.

He added that America’s nuclear plants have undergone exhaustive study and are safe, but he asked for a comprehensive review of all America's nuclear plants to make sure they are prepared for any future catastrophes.

He also continued to pledge American assistance and aid to Japan through this crisis.

Meanwhile, the first evacuation flight of U.S. citizens left Tokyo for Taiwan.

Undersecretary of State Patrick Kennedy said fewer than 100 people were on board. Most were dependents of U.S. officials, but some were private citizens.

Meanwhile, Japanese emergency workers were trying to regain control of the nuclear power complex in Fukushima by dousing it with water from police cannons, fire trucks and helicopters, to cool the nuclear fuel rods.

U.S. officials say it could take weeks to bring the crippled nuclear complex under control, and they warn Americans to not to go to Japan except for essential business.