(Adds French and British envoys, diplomats; paragraphs 7-15)

By Louis Charbonneau

UNITED NATIONS, May 12 (Reuters) - U.N. Secretary-General Ban Ki-moon ratcheted up the pressure on Myanmar on Monday, saying he was extremely frustrated by the junta's slow delivery of aid to more than 1.5 million victims of Cyclone Nargis.

"Today is the eleventh day since ... Nargis hit Myanmar," Ban told reporters. "I want to register my deep concern -- and immense frustration -- at the unacceptably slow response to this grave humanitarian crisis."

In his most critical comments on Myanmar's military government to date, Ban said that despite repeated attempts to contact the junta's senior general, Than Shwe, he had been unable to speak with him and had sent him a letter.

"We are at a critical point," he said. "Unless more aid gets into the country very quickly, we face an outbreak of infectious diseases that could dwarf today's crisis."

"I therefore call, in the most strenuous terms, on the government of Myanmar to put its people's lives first. It must do all that it can to prevent this disaster from becoming even more serious."

U.N. humanitarian affairs chief John Holmes told reporters a problem with visas for U.N. relief officials had improved somewhat. He said a total of 34 Myanmar visas were being granted to U.N. aid workers, though more would be needed.

France's Deputy U.N. Ambassador Jean-Pierre Lacroix raised the issue at a meeting of the Security Council. He told reporters that Paris fully supported Ban's statement about the government of Myanmar, formerly known as Burma.

"We share that frustration," he said. "If there is no progress (on aid delivery)...we will again raise that issue in the Security Council and will consider submitting a text."

Western diplomats said the French were considering asking the 15-nation council to adopt a nonbinding statement calling on Myanmar to lift all restrictions on foreign aid workers.

But if Myanmar continues to ignore international appeals, Paris might want the council to go a step further and adopt a legally binding resolution, the diplomats said, adding that the council might return to the issue on Tuesday.

BRITAIN, FRANCE BACK BAN'S CRITICISM

Last week, France called on the council to invoke a U.N. concept known as the "responsibility to protect" to authorize foreign delivery of aid shipments to Myanmar's population without the junta's authorization.

However, a number of countries, including Britain, said this concept was conceived for situations like genocide or war crimes, not natural disasters.

British Ambassador John Sawers, the Security Council's current president, said some members felt Myanmar's humanitarian crisis was not an appropriate issue for the council. Diplomats say China, Russia, Vietnam, South Africa and Indonesia were among those.

Western members of the Security Council strongly disagree.

"The cyclone there 11 days ago took tens of thousands of lives, but the inability to get aid through is risking hundreds of thousands of lives," Sawers said.

He also condemned Myanmar senior general Than Shwe's refusal to speak with Ban Ki-moon.

"I think it's shocking, frankly, that at a time of natural disaster like this, when the whole international community under the leadership of the United Nations is lined up to help, that the leaders of that country are not prepared to engage in discussions as to how that help can best be deployed." (Reporting by Louis Charbonneau, editing by Chris Wilson)

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