WASHINGTON, May 22 (Reuters) - Myanmar's decision to allow a U.S. aid official to tour the Cyclone Nargis disaster zone marked a small opening by the military government, but real help for millions of victims requires more expert access, U.S. officials said on Thursday.

William Berger, head of a U.S. Disaster Assistance Response Team for Myanmar, had joined other donor government representatives and aid workers for a three-day Myanmar government-organized tour of the Irrawaddy Delta area.

"We see this as an opening, but it is not sufficient," said Ky Luu, director of foreign disaster assistance at the U.S. Agency for International Development (USAID).

He told reporters that reaching all of the estimated 2.4 million people affected by the cyclone would require "tripling or quadrupling" aid experts and relief supplies.

U.S. State Department spokesman Tom Casey said Berger would use any meetings with officials of the former Burma to press for wider access for outside experts to assess needs.

"We're glad that he's able to at least go and make some small observations on the situation, but that certainly isn't the equivalent of him either being allowed in on his own or collectively with the DART team to do what they would normally do in this situation," he said.

While Berger visited Myanmar, the rest of his 10-member team of relief experts remained in Thailand awaiting visas, another official said.

U.S. emergency relief, which totaled $20.5 million as of Wednesday, would continue to flow to Myanmar through NGO partners and the United States would attend a weekend donors conference in Myanmar, Casey said.

But he added: "Before we can really offer any kind of additional assistance, we and other donors need some kind of independent assessment of the situation."

Myanmar's secretive ruling military has limited access by outside experts to the disaster zone since the May 2 storm and sea surge left nearly 134,000 dead or missing, but some foreign aid groups have been working with their local staff and community volunteers to reach victims.

"People are getting supplies to a fraction of the entire group of affected people -- about 25 to 30 percent of the affected people," said International Rescue Committee Chairwoman Anne Richard.

"We are getting people in but we have to get more experts in," she told reporters in Washington. (Reporting by Paul Eckert; Editing by Eric Walsh)

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