Russia said on Tuesday that unspecified external forces were undermining envoy Kofi Annan's peace effort in Syria and support for government foes posed a threat to a fragile ceasefire.
Opponents of peace had consigned Annan's plan to end 13 months of violence in Syria to failure and "are doing a lot to see to it that this prophecy comes true", Foreign Minister Sergei Lavrov said in televised remarks.
"They are doing this by delivering arms to the Syrian opposition and stimulating the activity of rebels who continue to attack both government facilities and ... civilian facilities on a daily basis," he said, without giving specifics.
"Of course, government forces are also taking measures to react to such provocations," said Lavrov, whose country is one of the strongest remaining allies of Syrian President Bashar al-Assad.
Members of the National Coordination Body, a centrist Syrian opposition group on a visit to Moscow, said they shared Russia's opposition to foreign intervention but maintained that use of force by the opposition was entirely defensive.
"We think the violence that the opposition has turned to - violence from the people, so to speak - is a reaction to the violence from government armed forces and that it will stop as soon as the pressure from the regime stops," Haitham Manaa said at a news conference.
Abdulaziz Al Khaier, another member of the delegation which was to meet with Lavrov, said Russia "has a very strong position of influence to convince its partners to stop the violence".
UN-Arab League mediator Annan's plan calls for political dialogue among Syrians, but Khaier said the group could only engage in dialogue if Assad proved he was not responsible for the bloodshed.
"It is unacceptable to sit at the table with representatives of the Syrian government whose hands are sullied with the people's blood," Khaier said through an interpreter.
"If (Assad) wants to show that he is not responsible for what happened and that his hands are clean, let him make such statements and go before the appropriate bodies.
"But until this happens ... he continues to be responsible for all this bloodshed, all this slaughter in our country."
Russia has provided weapons
Lavrov reiterated Russia's calls for foreign countries to press Assad's opponents to comply with Annan's plan, and its criticism of the "Friends of Syria" group of Western and Arab nations, which he said was undermining UN peace efforts.
"There are countries, there are external forces, that are ... encouraging the Syrian opposition not to cooperate with the government in providing for a ceasefire and the subsequent establishment of dialogue," Lavrov said.
Moscow has pledged its full support for Annan's peace plan and last week called on the Syrian government to step up implementation, but also has put much of the blame for the bloodshed on opposition forces.
Moscow says NATO used a UN resolution authorizing intervention in Libya to help rebels overthrow Muammar Gaddafi last year and has vowed not let it happen in Syria.
Russia has provided Syria with weapons and shielded Assad by blocking UN Security Council resolutions condemning his government for a crackdown in which the United Nations says its forces have killed more than 9,000 people since March 2011.
Syria has given post-Soviet Russia its firmest foothold in the Middle East, buying billions of dollars' worth of weapons and hosting a maintenance and supply facility that is Russia's only warm-water naval port outside the former Soviet Union.
Meetings with opponents and criticism of Assad indicate Moscow is hedging its bets and manoeuvring to preserve influence in Syria in the event Assad is forced out.
In another sign that Moscow is keeping its options open, Deputy Foreign Minister Sergei Ryabkov said Russia would be "weightily represented" in a UN observer mission charged with overseeing an end to the violence.