(Adds Brahimi, other quotes)
* Opposition says Alkhatib will meet senior U.S. and Russian officials
* But Russia says no meeting currently in Lavrov's schedule
* Alkhatib fights off internal challenge after talks offer
* Question of Assad's departure divides U.S., Russia, opposition
By Khaled Yacoub Oweis
AMMAN, Feb 1 (Reuters) - Moscow and the United Nations played down Syrian opposition assertions that its leader would hold a joint meeting with the U.N. Syria envoy and officials from the United States and Russia at a security conference in Munich on Saturday.
But a Russian diplomatic source did not rule out a meeting taking place 'spontaneously' at the weekend Conference on Security and Cooperation in Europe in the southern German city.
If it occurred it would be the first time the United States and Russia, who have been at loggerheads over whether President Bashar al-Assad can have a role in a transitional government, had sat down together with the opposition.
Opposition sources had said early on Friday that National Coalition President Moaz Alkhatib would meet Russian Foreign Minister Sergei Lavrov, U.S. Vice President Joe Biden and U.N. envoy Lakhdar Brahimi on the sidelines of the conference.
But 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," a U.N. official in Munich said, though Brahimi did plan separate meetings with Biden, Alkhatib and Lavrov.
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.
Lavrov has yet to meet Alkhatib, who in December rejected a Russian invitation to come to Moscow for talks and urged Lavrov to apologise for what he said was Russian intervention in Syria and support for Assad.
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 tens of thousands of political prisoners arrested since the revolt began were freed, without Assad stepping down first.
A high-level Coalition member told Reuters Alkhatib's changed position is what softened the resistance of the Russians to a meeting.
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".
Brahimi likened cities in Syria to the battered German capital Berlin at the end of World War Two.
Citing the "mass murder" of nearly 100 students at Aleppo University, the "slaughter" of civilians in Homs province and the apparent execution of 65 men in an Aleppo suburb - all in the last month - Brahimi said Syria's "tragedy simply does not have an end".
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.
A Western diplomat in contact with the Syrian opposition said any proposals made in Munich had to be crafted carefully if Alkhatib was going to be able to accept without losing credibility.
"The level of frustration among the opposition with lack of international support for the revolt is very high," the diplomat said. "We may be seeing one of the last chances before the opposition says: 'To hell with the international community, and let's direct all our efforts toward war'."
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.
"Therefore, I think it's incumbent on those nations that have refused to be constructive players to reconsider their positions because the worst kind of predictions of what could happen internally and spilling over the borders of Syria are certainly within the realm of the possible now."
Western support for the uprising has been mostly limited to political rhetoric denouncing Assad and humanitarian aid, despite pleas from rebels for arms.
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. (Writing by Oliver Holmes; Editing by Sonya Hepinstall)