Syrian government troops have shelled neighbourhoods of Damascus after clashes with rebel fighters, who also launched deadly attacks on the military elsewhere in the country, as the new UN envoy held meetings at the United Nations headquarters in New York.
The Britain-based Syrian Observatory for Human Rights, an activist group, said the bombardment of the southern neighbourhood of Tadamon in the capital early on Saturday followed street fighting with rebels overnight on Friday.
Another activist group, the Local Coordination Committees, said troops also shelled the nearby neighbourhood of Hajar Aswad.
Syrian regime forces have been able to recapture most areas of the capital that rebels seized in a July offensive, though opposition fighters continue to stage attacks using hit-and-run tactics in areas where they enjoy popular support, and recently claimed to have shot down a helicopter.
Meanwhile, gun battles between Syrian troops and opposition forces were being fought in the country's biggest city, Aleppo.
The town of Azaz, north of Aleppo, also faced intense bombardment, with many areas already reduced to rubble, according to residents. Azaz was taken over by the Free Syrian Army earlier this year, and has been under almost continual bombing by government forces.
On Saturday, there were also reports of new fighting in Homs, where an explosion occurred in a heavily built up area of the city. Parts of Homs have remained under siege by government forces for the last three months.
In Abu Kamal on the Iraqi border, rebel fighters captured the main air defence building, the Observatory said, adding that "preliminary reports" suggested they had seized ground-to-air missiles that could boost their ability to down government aircraft.
The assault late on Friday came after a rebel attack on the Abu Zohur airbase in Idlib province in the northwest, where the Free Syrian Army said it downed a MiG warplane shortly after takeoff earlier this week.
In their assault in Abu Kamal, rebel fighters also captured 16 air defence personnel and attacked the nearby Hamdan airbase, the Observatory said.
The seizure of the defence headquarters was a "major coup" for the rebels, the Britain-based watchdog's director Rami Abdel Rahman told AFP news agency, adding that it sparked retaliatory shelling in the town of some 60,000 people that killed at least five civilians.
They were among a total of 125 people killed in violence nationwide on Friday - 74 civilians, 29 soldiers and 22 rebels, according to the Observatory's figures.
The rebels claim to have destroyed a dozen aircraft on the ground in their attacks on air bases in recent days as they seek to counter the government's use of MiG warplanes and helicopter gunships against them.
Russia not 'holding onto regime'
With the insurgency intensifying, Russian Foreign Minister Sergei Lavrov said calls by Western and Arab governments for President Bashar al-Assad to unilaterally pull back his troops amounted to a demand for "capitulation" that they had no right to make.
In talks with Syrian Prime Minister Wael al-Halaqi in Tehran on Friday, UN secretary-general Ban Ki-moon said that Assad's regime must stop using its heavy weapons.
But Lavrov said on Saturday that such calls were "completely unrealistic".
"When our partners say that the government must stop first and withdraw all its soldiers and weapons from cities, and only then call on the opposition to do the same, well, this is a completely unworkable scheme," he said.
"Either people are naive or it is some sort of provocation," he added.
In-depth coverage of escalating violence across Syria
Lavrov stressed that Moscow, an Assad ally that maintains a military base in the coastal city of Latakia, was not trying to support Assad or his government but basing its policies on the daily situation on the ground.
"We are not holding on to any regime or any individuals in the Syrian situation. We are simply basing our position on what is realistic," he said.
Both Damascus and Aleppo, the largest city, have seen persistent fighting between troops and rebels in a conflict that has now claimed more than 26,000 lives since March last year, according to the Observatory's figures.
Turkey has been pressing for the establishment of safe havens inside Syria to stem the mounting exodus of refugees, and reacted with frustration when its calls fell on deaf ears at the UN Security Council on Thursday.
But on Friday Turkish Prime Minister Recep Tayyip Erdogan acknowledged that any such move would require UN backing and would be far too risky without the prior establishment of a no-fly zone.
Turkey is home to more than 80,000 refugees and thousands more have been stranded on other side of the border waiting to be accommodated in camps yet to be built by the Turks.
Meanwhile, the new UN-Arab Envoy to Syria is has been holding meetings at the United Nations headquarters in New York.
Lakhdar Brahimi will officially take over the position on Tuesday after Kofi Annan resigned in early August.
Brahimi told Al Jazeera that "change [in Syria] is necessary, indispensable, unavoidable".
"What kind of change and what phases that change will take place is something we are going to discuss," he said.
Brahimi spoke to Al Jazeera's Kristen Saloomey. She said Brahimi said that there is little he can do, apart from hear what both sides had to say.
The former Algerian diplomat and the head of the UN, Ban Ki-moon plan to discuss the Syrian conflict at the general assembly afterwards.