NAIROBI, Dec 3 (Reuters) - Ethiopia has a massive military force of 15,000 men in Somalia and will be to blame for any war in the chaotic Horn of Africa state, the speaker of parliament in Somalia's interim government said on Sunday.

Sharif Hassan Sheikh Adan rejected a U.S. draft resolution before the United Nations that would relax an arms embargo in order to let regional peacekeeping forces enter Somalia, saying it could jeopardise talks between his government and rival Islamists.

Adan, one of the top leaders of the weak Western-backed Somali administration, has been making efforts to reconcile it with the newly powerful Somalia Islamic Courts Council (SICC).

He was scathing about Ethiopian Prime Minister Meles Zenawi, who he said was destabilising the country by sending in a "massive military force" of men and equipment.

"Estimates by experts say there are around 15,000 (Ethiopian) troops now in Somalia," Adan told Reuters in an interview.

"They are not just sending a fighting force but the families of these fighters as well ... If war takes place now, it will be the responsibility of Ethiopia and its prime minister."

Addis Ababa is the main backer of Adan's fragile government, which has been confined to the provincial town of Baidoa by the rapid territorial gains of the Mogadishu-based Islamic courts.

Ethiopia denies sending any troops over the border, and says it only has several hundred armed military trainers in Somalia.

On Thursday, parliament in Addis Ababa voted to let Meles' government take "all necessary" steps to any Islamist invasion.

"ETHIOPIA MUST WITHDRAW"

Adan said Ethiopia had a free hand in Somalia for years, selling weapons to warlords whose militias carved up the country following the fall of former dictator Siad Barre in 1991.

"To avert war, Ethiopia must withdraw its troops without conditions and support the ongoing negotiations between the Somali sides to create peace," Adan said through a translator.

He flew to Mogadishu last month to meet the Islamists to try to restart Arab League-sponsored talks between them and his government that collapsed on Nov. 1 in Sudan's capital Khartoum.

But the deal he agreed with the SICC -- to resume discussions on political and security issues and power-sharing -- was rejected the next day by his own cabinet, which said it saw his meetings with the Islamists as a "personal" affair.

Adan, whose good relations with the SICC and some of their businessmen backers has put him at odds with President Abdullahi Yusuf and Prime Minister Ali Mohamed Gedi, said his position allowed him to negotiate in the "best interest" of all Somalis.

At the time, diplomats hailed the speaker's backchannel initiative as the best chance Somalia had of avoiding war, which experts fear could suck in the armies of neighbouring states.

Adan said he was hopeful negotiations between the two sides would resume in Sudan in mid-December.

But he echoed the concerns of European experts that a draft proposal by the United States at the U.N. Security Council to let east African peacekeepers enter Somalia put that at risk.

Talks about the arms embargo should come later, he said.

"The timing of this proposal is not right ... It is not possible to solve Somalia's crisis by military means," he said, adding that if the U.N. adopted the resolution, it would be seen as legitimising the Ethiopian military presence in Somalia.

"The next Khartoum talks are only 10 or 15 days away. Unless we make sure they resume, the worst consequences will happen."