* Opposition demands government agree Assad must go
* Syrian govt insists president is staying put
* Hope exists but it's fragile - France
* UN's Ban calls for immediate humanitarian access
* Russia, United States at odds over Assad's fate
By Mariam Karouny and Khaled Yacoub Oweis
MONTREUX, Switzerland, Jan 22 (Reuters) - Syria's government and opposition, meeting for the first time, vented their mutual hostility on Wednesday at a U.N. peace conference where world powers also offered sharply differing views on forcing out President Bashar al-Assad.
Opposition leader Ahmed Jarba accused Assad of Nazi-style war crimes and demanded the Syrian government delegation at the one-day meeting in Switzerland sign up to an international plan for handing over power.
Syrian Foreign Minister Walid al-Moualem insisted Assad would not bow to outside demands, graphically describing what he called atrocities of the "terrorists" - rebels supported by the Arab and Western states which were present in the room.
"Assad isn't going," Syria's information minister said.
The United States and Russia, co-sponsors of the conference which U.N. officials hope will lead to negotiations in Geneva from Friday, also revealed their differences over Assad during a day of formal presentations in Montreux on Lake Geneva.
The talks reflect global concern that a civil war which has killed over 130,000 and made millions homeless is spilling beyond Syria and encouraging sectarian militancy abroad.
There was little sign that any party was ready to make concessions at the meeting, which ended in the late afternoon.
Western officials were taken aback by the combative tone of Moualem, who also defied United Nations Secretary General Ban Ki-moon's plea to shorten his speech in testy exchanges.
"Hope exists but it's fragile. We must continue because the solution to this terrible Syrian conflict is political and needs us to continue discussions," said French Foreign Minister Laurent Fabius. "Obviously when we hear Bashar al-Assad's representative, whose tone is radically different, we know it will be difficult."
Moualem called on foreign powers to stop "supporting terrorism" and to lift sanctions against Damascus.
Referring to rebel acts, he said: "In Syria, the wombs of pregnant women are cut open, the foetuses are killed. Women are raped, dead or alive ... Men are slaughtered in front of their children in the name of the revolution."
He insisted Assad's future was not in question, saying: "Nobody in this world has a right to withdraw legitimacy from a president or government ... other than the Syrians themselves."
U.S. Secretary of State John Kerry echoed the rebel view that there is "no way" Assad can stay under the terms of a 2012 international accord urging an interim coalition. But Russian Foreign Minister Sergei Lavrov said all sides had a role and condemned "one-sided interpretations" of the 2012 pact.
Saudi Arabia, which backs the Sunni rebels, called for Iran and its Shi'ite Lebanese ally Hezbollah to withdraw forces from Syria. Iran, locked in a sectarian confrontation across the region, was absent, shunned by the opposition and the West for rejecting calls for a transitional government. Its president said Tehran's exclusion meant talks were unlikely to succeed.
The conference has raised no great expectations, particularly among Islamist rebels who have branded Western-backed opposition leaders as traitors for even taking part.
But even Western officials said hopes of talks in Geneva on Friday and beyond may be in jeopardy: "It's very far from encouraging," said a French diplomatic source. "We have the impression the regime has come to Geneva to ensure it fails."
German Foreign Minister Frank-Walter Steinmeier called Moualem's rejection of any government responsibility for the crisis "astounding and infuriating". Progress with Damascus was at risk, he said, "if they don't show some intelligence".
U.N. chief Ban opened proceedings by calling for immediate access for humanitarian aid convoys to areas under siege.
"After nearly three painful years of conflict and suffering in Syria, today is a day of fragile but real hope," Ban said, condemning human rights abuses across the board. "Great challenges lie ahead but they are not insurmountable."
But there was little sign of compromise on the central issue of whether Assad, who inherited power from his father 14 years ago, should make way for a government of national unity.
He himself says he could win re-election later this year and his fate has divided Moscow and Washington. Both endorse the conclusions of the 2012 meeting of world powers, known as Geneva 1, but differ on whether it means Assad must go now.
Opposition leader Jarba called for the government delegates to turn against their president before so-called Geneva 2 negotiations start: "We want to make sure we have a partner in this room that goes from being a Bashar al-Assad delegation to a free delegation so that all executive powers are transferred from Bashar al-Assad," the National Coalition leader added.
"My question is clear. Do we have such a partner?"
Lavrov repeated Moscow's opposition to "outside players" interfering in Syria's sovereign affairs and prejudging the outcome of talks on forming an interim government. He also said Iran - Assad's main foreign backer - should have a say.
The Kremlin is wary of what it sees as a Western appetite for toppling foreign autocrats that was whetted in Libya in 2011. Moscow opposes making Assad's departure a condition for peace. Speaking of the Geneva Communique, Lavrov said: "The essence of this document is that mutual agreement between the government and opposition should decide the future of Syria."
Kerry also spoke of "mutual" agreement among Syrians, but in a sense that excluded Assad.
"We see only one option - negotiating a transition government born by mutual consent," he said. "That means that Bashar al-Assad will not be part of that transition government."
Despite the differences, however, some participants believe common interests in reining in violence could rally the West, Russia and possibly even Iran behind some form of compromise.
A last-minute invitation from Ban to Iran was revoked after the Syrian opposition threatened to boycott the talks - a move that threatened to undermine months of U.S. and Western efforts to cajole Jarba's National Coalition into taking part.
President Hassan Rouhani said from Tehran that Iran's exclusion made it unlikely the conference could succeed.
WAR RAGES IN SYRIA
During the speeches in Montreux, the war went on in Syria.
The Syrian Observatory for Human Rights monitoring group reported clashes and air strikes around the country. Around Damascus, government artillery hit villages and rebels clashed with the army in the neighbourhood of Jobar on the northeast fringe of the capital, it said. Activists also reported clashes in Hama, Aleppo and the southern province of Deraa.
The release of photographs apparently showing prisoners tortured and killed by the government was cited by Jarba and Western ministers. The Syrian government rejected the report as not objective and aimed at undermining negotiations.
Discontent stretches back to the rule since 1970 of Assad's father, who took power in a military coup, but it boiled over in March 2011 as Syria's drought-hit economy struggled and the Arab Spring uprisings in Tunisia and Egypt inspired protests.
When those were crushed, the revolt became a war that has taken on an increasingly sectarian complexion, setting majority Sunnis against Assad's Alawite community, an offshoot of Shi'ite Islam.
Militants linked to al Qaeda and other Islamists have become the most powerful forces on the rebel side, dampening Western appetite for direct intervention and provoking conflict among rival rebel groups. Iran and Hezbollah have helped Assad. And violence has spread, notably to Iraq and Lebanon.
In Damascus, where life limps on amid bombardments and checkpoints, weary residents cautiously hope for better.
"I really don't think much will come out of it, but the alternative is no talks at all, and that's not much better," said Ruba, a mother of two. (Additional reporting by Tom Miles, Gabriela Baczynska, Dominic Evans, Samia Nakhoul, John Irish, Stephanie Nebehay, Lesley Wroughton and Johnny Cotton in Montreux Guy Faulconbridge in London and Laila Bassam, Paris Hafezi in Ankara, Alexander Dziadosz, Oliver Holmes and Stephen Kalin in Beirut; Writing by Alastair Macdonald and David Stamp, editing by Peter Millership)