* US ambassador Rice says D'Escoto appointment dubious
* D'Escoto must leave US, apply for new visa, Rice says
UNITED NATIONS, March 29 (Reuters) - The U.S. ambassador to the United Nations cast doubt on Wednesday on the naming of a former Nicaraguan foreign minister as Libyan envoy to the world body, saying he must leave the United States and re-apply for a visa.
Nicaragua said on Tuesday that Miguel D'Escoto Brockmann, who once served in a leftist Sandinista government and has been a sharp critic of U.S. governments, would represent Libya at the United Nations.
Most diplomats at the Libyan U.N. mission, including the ambassador and his deputy, defected from the government of Muammar Gaddafi last month and sided with rebels who launched an uprising in eastern Libya.
Tripoli then appointed senior diplomat Ali Treki, a close associate of Gaddafi, as its U.N. envoy but he has not arrived in the United States.
U.S. Ambassador Susan Rice said D'Escoto had been appointed by former Libyan foreign minister Moussa Koussa, who defected to Britain on Wednesday saying, according to British officials, that he no longer wished to represent Gaddafi's government.
"So I think the first question is whether he's actually been appointed in any legitimate fashion that anybody needs to consider at this stage," Rice told reporters.
She said D'Escoto, while born in the United States, had renounced U.S. citizenship some years ago, and was currently in the United States on a tourist visa, which "does not allow you to represent any country ... at the United Nations."
If a legitimate representative of the Libyan government wished to re-nominate D'Escoto, the Nicaraguan would have to leave the United States and apply for a G1 visa -- the type given to foreign envoys, she said.
She hinted that if he acted as a government representative while on a tourist visa, that visa could be withdrawn.
The United Nations had said earlier in the day that D'Escoto, a former president of the U.N. General Assembly, would give a news conference on Thursday on U.N. premises to discuss Libya.
But minutes after Rice suggested to journalists it was inappropriate that "somebody who represents, seemingly, nobody" should hold a news conference in a U.N. facility, U.N. officials said the conference had been canceled.
D'Escoto once called former U.S. President Ronald Reagan "the butcher of my people" and angered U.S. officials with other statements while at the United Nations from 2008-09.
The Nicaraguan government of leftist President Daniel Ortega said on Tuesday D'Escoto has flown to U.N. headquarters in New York to "support our Libyan brothers in their diplomatic battle to enforce respect for its sovereignty."
D'Escoto was foreign minister in an earlier Ortega administration that ruled Nicaragua from 1979-90 and that fought an insurgency by U.S.-backed rebels. He was born in Los Angeles and ordained as a Roman Catholic priest.
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