* Russia suspects rebels staged Syria sarin gas attack
* Lavrov wants U.N. investigators to continue probe
* Blame game points to diplomatic challenges ahead
By Alissa de Carbonnel
MOSCOW, Sept 17 (Reuters) - Russia and France sharply disagreed on Tuesday over a report by U.N. investigators into a chemical weapons attack that killed hundreds of people in Syria, underscoring the difficulties in reaching agreement on action at the U.N. Security Council.
Sitting beside French Foreign Minister Laurent Fabius at a news conference in Moscow, Russian Foreign Minister Sergei Lavrov said the report had produced no proof that President Bashar al-Assad's troops carried out the Aug. 21 attack and that Russia still suspected rebel forces were behind it.
Fabius took the opposite view, saying the report left no doubt that Assad's forces were to blame for the attack which Washington says killed more than 1,400 people. The United States has also blamed Syrian government forces.
Lavrov acknowledged that the investigators' report proved that chemical weapons had been used but that "there is no answer to a number of questions we have asked," including whether the weapons were produced in a factory or home-made.
"We have very serious grounds to believe that this was a provocation," Lavrov said after talks in Moscow between two countries with veto powers in the U.N. Security Council.
He said there had been "many provocations" by the rebels fighting Assad's government, adding: "They were all aimed, over the last two years, at provoking foreign intervention."
Lavrov said the U.N. report should be examined not in isolation but along with evidence from sources such as the Internet and other media, including accounts from "nuns at a nearby convent" and a journalist who had spoken to rebels.
"We want the events of Aug. 21 to be investigated dispassionately, objectively and professionally," he said.
FRANCE BLAMES ASSAD'S FORCES
After Lavrov spoke, Fabius, whose country has stood with U.S. President Barack Obama in backing military action against Syria, challenged Lavrov's interpretation by saying the result of the report was clear.
"When you look at the amount of sarin gas used, the vectors, the techniques behind such an attack, as well as other aspects, it seems to leave no doubt that the regime is behind it," Fabius said.
The group Human Rights Watch said rocket trajectories detailed in the U.N. report suggested they had been fired from a base belonging to the Republican Guard, run by Assad's brother Maher.
Drawing lines of presumed rocket flight paths back from two sites on opposite sides of the Syrian capital that were struck on Aug. 21, it said they converged in the hills north of central Damascus where the Republican Guard 104th Brigade is based.
"The Sellstrom report revealed key details of the attack that strongly suggest the government is to blame, and may even help identify the location from which the sarin-filled rockets that killed hundreds of people ... were fired," it said.
"This isn't conclusive, given the limited data available to the U.N. team, but it is highly suggestive and another piece of the puzzle," the group said.
The United States also rejected Lavrov's interpretation.
"He is swimming against the tide of international public opinion," State Department spokeswoman Jen Psaki said in Washington. "The report confirms unequivocally that chemical weapons, including the nerve agent sarin, were used in Syria."
"Based on the preliminary review of information contained in the report several crucial details confirm the Assad regime's guilt in carrying out this attack," Psaki added.
Lavrov and Fabius agreed that there should be a renewed push for a political solution in Syria. Lavrov also thanked France for supporting a U.S.-Russian deal which calls for Syria to account fully for its chemical weapons within a week and for the removal and destruction of the entire arsenal by mid-2014.
But the differences over culpability for the Aug. 21 attack indicated the hurdles faced in turning the chemical weapons agreement into progress towards ending a civil war that has killed more than 100,000 people since March 2011.
LIKELY WRANGLING AHEAD
Their comments also pointed to likely wrangling in the U.N. Security Council over the mechanism for enforcing the agreement, which Assad has accepted, and disputes over punishment for any violations.
"This will be a litmus test for the future work of the Security Council," Lavrov said. "Either we grab for Chapter VII whenever someone says the regime or the opposition has used chemical weapons ... or we rely on professionals who must thoroughly and professionally examine each episode and claim."
A U.N. resolution under Chapter VII could authorise military intervention in Syria, which Russia has adamantly opposed.
Fabius, who went to Moscow to discuss a U.N. resolution that would frame the U.S.-Russian accord, said the agreement was an "important step forward but not the end of the story".
"There is a series of precise mechanisms that have to be placed into a U.N. decision. We spoke about this and it should be dealt with in the coming days. I insisted, like Sergei Lavrov, on the necessity to go quickly," he said.
Lavrov cautioned that while the Security Council was to adopt a resolution supporting the chemical weapons deal, a separate resolution would be needed to authorise use of force in response to any new attack and after guilt was proven.
Along with diplomatic ally China, Russia has used its veto power three times to block Western-backed Security Council resolutions meant to push Assad out or muscle him into ending a conflict that began with a crackdown on protests.
Lavrov said putting too much pressure on Assad would encourage his opponents and hurt the chances for peace.
Fabius suggested it was Assad who should be kept in line and that he must keep his promise to abandon chemical arms.