(Adds Bush criticism in paragraphs 2-4)

By Susan Cornwell and Paul Eckert

WASHINGTON, May 12 (Reuters) - The United States will send two more aid flights to cyclone-stricken Myanmar and offer $13 million more in aid through U.N. agencies even though U.S. officials involved in the relief effort have not been allowed beyond Yangon airport, U.S. officials said on Monday.

As the first U.S. aid flight arrived in Myanmar on Monday, U.S. President George W. Bush condemned the country's military junta for failing to act more quickly to accept international help after Cyclone Nargis devastated the country, saying "either they are isolated or callous."

"It's been days and no telling how many people have lost their lives as a result of the slow response," Bush said in a radio interview with CBS News. "An American plane finally went in but the response isn't good enough."

Henrietta Fore, administrator of the U.S. Agency for International Development, and Adm. Timothy Keating, head of the U.S. Pacific Command, flew to Myanmar on Monday with the first C-130 cargo planeload of U.S. supplies, then watched as it was transferred to Myanmar helicopters they were told would fly to stricken areas, a U.S. official said.

"While on the ground, Administrator Fore and Admiral Keating witnessed helicopters carrying the U.S. cargo bound for Bogalay township," Ky Luu, director of USAID's Office of Foreign Disaster Assistance, told reporters.

Because U.S. officials have serious concerns over whether the aid would reach cyclone victims, they will contact nongovernmental organizations in Bogalay to verify the supplies in fact arrived there, he said.

At the White House, spokeswoman Dana Perino told reporters the United States would provide $13 million in food and logistical assistance for U.N. World Food Program relief operations in the former Burma, bring the total U.S. aid to $16.25 million.

"We will send two flights of relief supplies tomorrow and then we'll take it one day at a time from there," she said.

"It's a drop in a bucket for what they're going to need and we would hope that the Burmese junta would allow more flights to come in," Perino said, adding that the president was "very concerned about it."

WAITING FOR VISAS

The first U.S. C-130 cargo plane that flew into Yangon from Thailand on Monday carried 8,300 bottles of water, 1,350 blankets and 10,800 mosquito nets, which will help as many as 30,000 people, Luu told reporters at the State Department.

The United Nations says 1.5 million people are in need of immediate assistance from the cyclone, in which up to 100,000 people are feared dead.

Fore, Keating and Bill Berger, the leader of a U.S. disaster relief team that has been waiting unsuccessfully for a week to get visas for Myanmar, met Burmese authorities at the airport and urged them to "open up access, to issue additional visas and to bring in additional supplies," Luu added.

The Americans were at the airport for about two hours, he said.

"I think we have to stay optimistic on this," Luu said, while acknowledging that there was "massive concern" about whether the aid would in fact reach cyclone victims.

While Myanmar's reclusive military government is accepting aid from the outside world, including the United Nations, it will not let in foreign logistics teams, who were queuing up in Bangkok hoping to get visas from the Myanmar Embassy.

U.S. officials had been told last week that the Myanmar government "would accept our commodities but not accept our disaster experts," Luu said.

Keating had said before taking off that he would urge the junta to allow a "long, continuous train of flights" that could carry up to 200,000 pounds (90,720 kg) of relief goods a day.

The new assistance would include $12 million for food aid that will arrive in the coming weeks, including 1,000 tons of urgently needed food from a U.S. AID Food for Peace warehouse in Djibouti in Eastern Africa, Perino said.

(Additional reporting by Jeremy Pelofsky; writing by Susan Cornwell; editing by David Alexander and Jackie Frank)

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