* AU wants to boost force from 7,200 to 20,000
* U.N. no-fly-zone, naval blockade seen unlikely soon
By Patrick Worsnip
UNITED NATIONS, Oct 21 (Reuters) - U.N. Security Council countries voiced support on Thursday for boosting the African Union peacekeeping force in conflict-torn Somalia, but told the AU to provide more details of its plans first, diplomats said.
The AU wants to step up its AMISOM force to 20,000 from the current level of 7,200 but needs funding from the United Nations and Security Council authorization. It also wants the council to impose a no-fly-zone and naval blockade on Somalia.
Two Islamist militant groups have waged a three-year insurgency to topple Somalia's Western-backed government. An AU envoy said earlier this month that AMISOM, composed of troops from Uganda and Burundi, was making progress against the rebels in the capital, Mogadishu.
Following a closed-door council meeting, diplomats said the 15-nation body was not opposed to expanding the force in the Horn of Africa nation. But one Western envoy said the AU had not so far substantiated the need for its requests.
"We need to have more concrete information. They have been told to go back and justify what they are saying," the diplomat said.
AU Peace and Security Commissioner Ramtane Lamamra, who attended the meeting, later told reporters the council "wanted to know how, and which troops to achieve which political objectives, so they cannot just give us blanket (approval)."
Uganda has said it is willing to supply all the troops to build AMISOM up to 20,000. But the Western diplomat described that figure as "quite excessive" and predicted a bargaining process with the AU to agree on the exact figure and costs.
AMISOM is already funded for $130 million a year through a U.N. support office, diplomats said.
U.S. Assistant Secretary of State Johnnie Carson said on Wednesday that Washington supported an increase in the number of peacekeepers in Somalia but took no position on what that number should be.
Lamamra conceded the request for a no-fly-zone and naval blockade -- aimed at shutting off the flow of arms and recruits to the rebels -- was unlikely to be agreed on soon. "There is support, but not to get it this time," he said.
But he said he hoped the Security Council would pass a resolution next month to authorize an increase in AMISOM and that the extra troops could start arriving in February.
A Security Council diplomat said the body wanted "to move quite quickly" and could take action in "weeks rather than months."
The AU has long been pushing the United Nations to send its own peacekeepers or to "rehat" AMISOM as a U.N. force. The Security Council, already deploying large forces in Congo and Sudan, has been leery of getting involved in Somalia, where an earlier peacekeeping effort came to grief in the 1990s.
Addressing the council in a public section of Thursday's meeting, Lamamra criticized the international community's policy of "limited engagement and half-hearted measures, in the false hope that the situation can be contained."
His appeal for support for building up AMISOM was backed by U.N. Secretary-General Ban Ki-moon, who said advances against the rebels showed there were "glimmers of hope" in Somalia and called on the council to take "bold and courageous decisions."
A statement issued by the council after its meeting did not directly address the AU requests but called on the international community to provide more resources for AMISOM. (Editing by Peter Cooney)