Syria's opposition has refused to hold talks to end the country's escalating violence, after Russia said that Syrian authorities had said they were prepared to hold talks with the opposition in Moscow.
Russia, which has resisted Western calls to back UN sanctions against Damascus, had suggested to the government and the opposition that they should meet in the Russian capital for "informal contacts" without any preconditions.
"Our offer has already received a positive response from the Syrian authorities," Russia's foreign ministry said on Monday.
Russia continues to block a draft UN Security Council resolution calling for a transfer of power in Syria, where it maintains a naval base at Tartous, near Latakia.
The head of the opposition Syrian National Council said that the opposition rejected all such talks with Damascus until President Bashar al-Assad steps down.
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"The resignation of Assad is the condition for any negotiation on the transition to a democratic government in Syria," Burhan Ghalioun told the AFP news agency.
The Russian foreign ministry had warned the Syrian opposition that Moscow was counting on its participation in the talks.
"We are expecting that the opposition will also give their assent in the next days and put the interests of the Syrian people before any other ideas," it said.
Moscow's diplomatic moves came at a time of mounting concern that the clashes between the opposition and security forces have become even deadlier, with at least 160 people killed in the past two days, according to activists.
The UN has said that thousands of people have died since the uprising began in March.
Call for action
US Secretary of State Hillary Clinton said on Monday that the UN Security Council "must act" on Syria to end Assad's "violent and brutal attacks" against demonstrators.
Al Jazeera's Kristen Saloomey, reporting from the United Nations in New York, said that French diplomats are claiming that the resolution now has a majority of votes in the Security Council.
"All diplomatic muscles are being flexed here in New York in an attempt to win support for a resolution that is put forward by Morocco in the Security Council... But it comes down to China and Russia who have both expressed concern over the resolution."
Clinton said that she would travel to the United Nations on Tuesday to "send a clear message of support to the Syrian people: we stand with you".
"The United States condemns in the strongest possible terms the escalation of the Syrian regime's violent and brutal attacks on its own people," Clinton said in a statement.
"The Security Council must act and make clear to the Syrian regime that the world community views its actions as a threat to peace and security. The violence must end, so that a new period of democratic transition can begin," she said.
The Syrian National Council warned on Monday of a possible "massacre" of hundreds of young men rounded up by security forces in a town near Damascus.
It voiced "fears over a possible liquidation of hundreds of young men that Syrian security services have gathered in a public square in Rankus," 40km north of the capital, in a statement received in Nicosia.
Security forces "raided Rankus this morning, backed by tanks and rocket-launchers ... and launched a campaign of arrests," it said.
Meanwhile, fierce street battles were reported in many other suburbs of Damascus, where security forces were said to have pushed rebel fighters back from their positions in Douma, Saqba and Hamuriyeh.
An activist named Kamal, speaking to the Reuters news agency by telephone from the Al-Ghouta area on the eastern edge of the capital, said that security forces had re-occupied the suburbs.
"The Free Syrian Army has made a tactical withdrawal. Regime forces have re-occupied the suburbs and started making house-to-house arrests," he said. A spokesman for the Free Syrian Army appeared to confirm that account.
Gunbattles were also reported in the towns of Deraa and Bab Amr.
Meanwhile, state media said "terrorists" had blown up a gas pipeline in the central province of Homs near the border with Lebanon, causing a leak of about 460,000 cubic metres of gas.
Al Jazeera's Anita McNaught, reporting from neighbouring Turkey, said this was not the first attack on pipelines in Syria.
"There have been at least five previous attacks since the start of the uprising. Damaging infrastructure is very much part of the strategy of the opposition forces. [...] It seems everything is in play now, perhaps even for either side."
The escalation in violence comes days after the Arab League suspended its beleaguered observer mission in the country, where activists have been calling for Assad to step down since last March.
Arab League condemned
Russia on Sunday strongly criticised the Arab League decision to halt its observer mission in Syria, saying the situation demands additional deployment of monitors and not their suspension.
"We would like to know why they are treating such a useful instrument in this way," Sergei Lavrov, the Russian foreign minister, said during a visit to Brunei.
"I would support an increased number of observers," Lavrov said. "We are surprised that after a decision was taken on prolonging the observers' mission for another month, some countries, particularly Persian Gulf countries, recalled their observers from the mission."
Our correspondent said Russia's support for Asad's government remained crucial. “What we understand is until he feels Russia will stop backing him, Bashar al-Assad will tough it out,” she said.
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The Arab League suspended its observer mission on Saturday as the bloodshed in a crackdown on anti-government protests gathered momentum. Several hundred people have died in the past four days alone.
Lavrov said that he did not back those Western countries that said the mission was pointless and that it was impossible to hold dialogue with Assad's government.
"I think these are very irresponsible statements because trying to sabotage a chance to calm the situation is absolutely unforgivable," he said.
Syria also voiced its dismay and surprise over the Arab League decision to halt its observer mission.
"This will have a negative impact and put pressure on [Security Council] deliberations with the aim of calling for foreign intervention and encouraging armed groups to increase violence," Syria Television reported on Saturday.
Source: Al Jazeera and agencies