* Opposition says head will meet Russian, U.S. officials
* Russia says no meeting currently in Lavrov schedule
* Alkhatib fights off challenge after talks offer
* U.N. envoy makes "last appeal" for solution
Senior U.S., Russian and U.N. officials, along with the leader of the Syrian opposition, were all expected at a security conference in Munich on Saturday, providing a rare opportunity for talks to revive efforts to end the civil war in Syria.
Moscow and the United Nations, however, played down Syrian opposition assertions that its leader would hold a joint meeting with Russian Foreign Minister Sergei Lavrov, U.S. Vice President Joe Biden and U.N. envoy Lakhdar Brahimi in Munich.
Brahimi, who said he had bilateral meetings planned with Biden and Lavrov on the sidelines of the conference in the southern German city, told the conference on Friday he was pessimistic about an early solution to the conflict.
"I am much more conscious of the difficulties, of the country being broken down day after day, than I am of a solution," he said, speaking at a panel alongside Syrian opposition leader, National Coalition President Moaz Alkhatib.
But a high-level member of the Syrian opposition coalition told Reuters that Russia may be softening towards a meeting with Alkhatib after he said he was willing to hold talks with the government of President Bashar al-Assad.
A Russian diplomatic source did not rule out a meeting taking place "spontaneously" at the weekend Conference on Security and Cooperation in Europe.
If it occurred it would be the first time the United States and Russia, who have been at loggerheads over whether Assad can have a role in a transitional government, had sat down together with the opposition.
A U.N. official and a senior Russian diplomat said there no plans for them all to meet together. "The U.N. special envoy is not involved in any trilateral meetings," the U.N. official in Munich said.
Russian Deputy Foreign Minister Gennady Gatilov said Lavrov was not scheduled to take part in expanded talks.
"Media reports of a meeting in Munich in the format of Lavrov-Biden-Brahimi and Syrian opposition representative Alkhatib do not correspond with reality," he said.
Brahimi said neither the Syrian people, nor the countries of the region, were able to find a way to end the conflict.
"All that is left is the wider international world," he said, saying only the U.N. Security Council - divided over Syria - could find a solution. "You are the last appeal," he told the conference. "Please do your job."
Russia, Syria's main arms supplier, has blocked three U.N. Security Council resolutions on the 22-month-old Syrian conflict, which has killed more than 60,000 people.
Alkhatib had to fight off an overnight challenge to his authority after going against the opposition's established position by saying he was willing to sit with Syrian officials to discuss a transition if political prisoners arrested since the revolt began were freed, without Assad stepping down first.
He said on Friday in Munich that if this were needed to end the bloodshed in Syria, and on condition of prisoners being released, "we are ready to sit at the negotiating table with the regime".
Brahimi told the Security Council this week that he believed an international accord reached in Geneva last year, which was deliberately vague about Assad's future role, was "largely understood... (to mean) that the president would have no role in the transition".
STRATEGY FOR PEACE OR WAR
The Coalition's 12-member politburo met until 5 a.m. on Friday and instructed Alkhatib not to respond to any proposals made in Munich without consulting with them first.
The structure of the 70-member Islamist-dominated coalition, which was formed with Western and Arab backing in December, makes Alkhatib, a Sunni preacher from Damascus, a first among equals rather than an outright leader.
The opposition source said the "knives were out for Alkhatib" from Islamists on the politburo and from the Syrian National Council after his remarks, which Kamal al-Labwani, a secular leader and long-time political prisoner, said hurt the morale of the revolt.
Alkhatib responded that he was motivated by the plight of the prisoners, many of whom are in secret police dungeons, and made clear that he still believed Assad and his cohorts must eventually leave.
U.S. Secretary of State Hillary Clinton urged Assad's allies Iran and Russia on Thursday to rethink their positions, saying the conflict could still spill beyond its borders. More than 700,000 Syrians have fled into neighbouring countries.
She told reporters there were signs that Iran was sending more people and increasingly sophisticated weaponry to support Assad's war, in which he has lost swathes of rural areas but managed to hold on to the central parts of most major cities.
"I personally have been warning for quite some time of the dangers associated with an increasingly lethal civil war and a potential proxy war," Clinton told a small group of reporters.
Western support for the uprising has been mostly limited to political rhetoric denouncing Assad and humanitarian aid, despite pleas from rebels for arms.
Britain's call to amend an EU embargo on arms sales to Syria to help opponents of Assad met opposition in Brussels on Thursday when EU governments warned it could allow weapons to end up in the wrong hands.
As the war has become more sectarian, Western powers have become more wary of supporting the largely Sunni Muslim, and increasingly radicalised, rebellion against Assad, whose family have ruled the country for more than four decades and belong to the minority Alawite sect, an offshoot of Shi'ite Islam.
On Wednesday, Israeli jets struck what Syria said was a military research centre and diplomats said was a weapons convoy heading for Lebanon.
Syria warned of a possible "surprise" response. Lebanese residents reported that Israeli war planes were flying in Lebanese airspace on Friday, a common occurrence but more sensitive after the strike.