By Steve Gutterman and Oliver Holmes
MOSCOW/BEIRUT (Reuters) - Russia denounced U.N. investigators' findings on a poison gas attack in Syria as preconceived and tainted by politics on Wednesday, stepping up its criticism of a report Western nations said proved President Bashar al-Assad's forces were responsible.
Russia, which holds veto power in the U.N. Security Council, could cite doubts about proof of culpability in opposing future efforts by the United States, Britain and France to punish Syria for any violations of a deal to abandon chemical weapons.
Russian Foreign Minister Sergei Lavrov, who negotiated the deal with Washington, said on Tuesday the U.N. report had not dispelled Russia suspicion that rebels staged the attack to try to provoke Western military intervention in the civil war.
Deputy Foreign Minister Sergei Ryabkov, visiting Syria as warplanes struck rebel-held areas and clashes continued unabated on the ground, took Russia's criticism a step further.
"We are disappointed, to put it mildly, about the approach taken by the U.N. secretariat and the U.N. inspectors, who prepared the report selectively and incompletely," Ryabkov told the state-run Russian news agency RIA in Damascus.
"Without receiving a full picture of what is happening here, it is impossible to call the nature of the conclusions reached by the U.N. experts ... anything but politicized, preconceived and one-sided," said Ryabkov, who met Syrian Foreign Minister Walid al-Moualem late on Tuesday and Assad on Wednesday.
Assad's government gave Ryabkov what it said was evidence that rebels were behind the attack, and Lavrov said Russia would present it to the Security Council.
Lavrov has said the investigation was incomplete without examination of evidence from other sources, including postings on the Internet and an account from nuns, and that suspicions of chemical arms use after August 21 should also be investigated.
"We also have information that there have been many incidents like the one in August in Ghouta," Lavrov said, referring to the August 21 attack. "We will consider all this in the Security Council along with the report by the U.N. experts."
During Ryabkov's visit, Assad thanked for Moscow's support against what the Syrian leader called Western-backed "terrorism".
The report issued on Monday confirmed the nerve agent sarin was used in the August 21 attack but did not assign blame. Britain, France and the United States said it confirmed Syria's government was behind it rather than rebels.
The United Nations defended the report on Wednesday, saying the findings were "indisputable". "They speak for themselves and this was a thoroughly objective report on that specific incident," spokesman Martin Nesirky said.
However, the stark disagreement over blame may complicate discussions among veto-holding Security Council members - Russia, China, the United States, Britain and France - over a Western-drafted resolution to eliminate Syria's chemical weapons.
"We are surprised by Russia's attitude because they are calling into question not the report, but the objectivity of the inspectors," French Foreign Minister Laurent Fabius said in Paris on Wednesday.
"I don't think anybody can call into question inspectors that have been appointed by the U.N.," said Fabius, who met Lavrov in Moscow on Tuesday and said several aspects of the U.N. report clearly pointed to Syrian government involvement.
Russia has been Assad's most powerful backer during the conflict that has killed more than 100,000 people since 2011, delivering weapons and - with China - blocking Western efforts to use the Security Council's clout to pressure his government.
Moscow argues that the danger emanates from rebels, many of whom harbor militant Islamist ambitions for Syria that could ultimately pose a threat both to Russia, which is fighting against Islamist militants on its southern fringe, and the West.
In his meeting with Ryabkov, Assad voiced appreciation "for Russia's stances in support of Syria in the face of the vicious attack and ... terrorism which is backed by Western, regional and Arab forces", Syrian state news agency SANA said.
The draft Security Council resolution is intended to support a U.S.-Russian deal reached on Saturday calling for Syria to account for its chemical weapons within a week and for their destruction by mid-2014.
The accord was based on a Russian proposal announced by Lavrov on September 9 and accepted by Assad the following day.
The deal halted efforts by U.S. President Barack Obama to win Congressional approval for military action to punish Assad for the gas attack, which the United States says killed more than 1,400 people in rebel-held areas.
U.S. Secretary of State John Kerry called on Tuesday for a U.N. resolution with the strength to force Assad's compliance.
Diplomats said the current U.S.-British-French draft was written so that its provisions were under Chapter 7 of the U.N. charter, which covers Security Council authority to enforce its decisions with measures such as sanctions or force.
But Russia has made clear it believes authorization of the use of force would require a second resolution to be introduced if the Syrian government or its opponents are found to have violated the country's commitments on chemical weapons.
Rebels have called the focus on poison gas a sideshow, dismissed talk the chemical arms pact might herald peace talks and said Assad has stepped up an offensive with ordinary weaponry now that the threat of U.S. air strikes has receded.
Activists said on Wednesday that Assad's air force hit the neighborhood of Berze, a northeastern part of central Damascus, where rebels are trying to push further into the city. Assad holds central Damascus districts but has lost some suburbs.
The British-based Syrian Observatory for Human Rights, which says it provides objective information about casualties on both sides of Syria's war from a network of monitors, reported violence in nearly every Syrian province.
It said war planes hit parts of southern Deraa province where the protests that have escalated into a civil war started more than two years ago, and that rebels and government forces clashed in the major cities of Homs, Deir al-Zor and Aleppo.
In Idlib province, which borders Turkey, the Observatory cited activists as reporting the killing and burning of 11 civilians by the army. Reuters cannot confirm reports due to security and reporting restrictions.
(Additional reporting by John Irish in Paris; Editing by Alison Williams)