Fighting is continuing in the eastern suburbs of Damascus, according to activists, as Syrian security forces appeared to be re-asserting their control over the restive fringes of the country's capital.
Activists reported on Monday that the Free Syrian Army, armed defectors fighting against Syrian President Bashar al-Assad's rule, had launched scattered attacks on government troops in the district of Saqba, which was reported in recent days to have been under opposition control.
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.
The new offensive came as a senior member of the Syrian opposition said that the Syrian National Council had not received any formal invitation to attend talks with Syria's authorities in Russia, and would decline if one arrived.
Earlier on Monday, Russia's Foreign Ministry said the Syrian authorities had agreed to talks in Russia, and that it hoped the opposition would join the negotiations.
Meanwhile, state media said "terrorists" had blown up a gas pipeline in the central province of Homs, 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.
Tanks rolling in
Activists inside Damascus told our correspondent that tanks had rolled into Al-Ghouta, within 10km of the city centre.
“Activists say it is the fiercest violence they have witnessed in months," she reported. “There are fires burning all over Syria, some say almost too many for the army to deploy all over the place."
Activists said there were 64 people killed in fighting across the country on Sunday, including three children and two defected recruits, according to the Local Co-ordination Committees (LCC), an activist network.
In the suburbs of Damascus, there were 16 deaths, in Kafar Batna, Saqba, Hamourya, Rankoos, Zabadany and Harasta.
At least 19 people were reported killed in Homs, 15 in Hama, five in Idlib, four in Deraa and three in Saraqeb, Deir Ezzor and Damascus, the LCC said.
At least 23 security forces were also killed, state TV reported.
The latest casualties add to the UN's toll of 5,400 people killed in the government's 10-month crackdown on protesters.
Amateur video appeared to show tanks rolling in to Al-Ghouta and Zamalka to the east amid unconfirmed reports that the shelling had destroyed homes with people inside.
In the southern province of Deraa, there were reports that security forces had killed two students when they broke into a school in the town of Jasim.
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.
In-depth coverage of escalating violence across Syria
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.
Nabil Elaraby, the Arab League chief, travelled to New York on Sunday seeking to win support from the UN Security Council for a plan to end violence in Syria by asking Assad to step aside.
"We will hold several meetings with representatives from members of the Security Council to obtain the council's support and agreement to the Arab initiative," Elaraby told reporters at Cairo airport shortly before leaving for New York.
The bloc said about 100 observers would remain in the country but would not undertake new missions.
The mission has been widely criticised by the Syrian opposition for failing to end the government's crackdown on protests.