Syrian rebels launched deadly attacks on the military on Saturday in a campaign increasingly targeting its air power, as President Bashar al-Assad's ally Russia said it was "naive" to expect him not to fight back.
Rebels captured the main air defence building in Albu Kamal on the Iraqi border, the Syrian Observatory for Human Rights said, adding "preliminary reports" suggested they seized ground-to-air missiles.
The assault late Friday came hot on the heels of a rebel attack on the Abu Zohur air base in Idlib province in the northwest, where the Free Syrian Army said it downed a MiG warplane this week.
With the insurgency intensifying, Russian Foreign Minister Sergei Lavrov said Western and Arab calls for Assad to unilaterally withdraw his troops amounted to a demand for "capitulation" that they had no right to make.
In their assault in Albu Kamal, rebels also captured 16 air defence personnel and attacked nearby Hamdan air base, the Observatory said.
The seizure was a "major coup" for the rebels, the Observatory's Rami Abdel Rahman told AFP, adding it sparked retaliatory shelling in the town of some 60,000 that killed at least five civilians.
They were among 125 people killed in nationwide violence on Friday, including 74 civilians, 29 soldiers and 22 rebels, said the Britain-based Observatory, making August the deadliest month of the conflict so far with nearly 5,000 dead.
The rebels claim to have destroyed a dozen aircraft on the ground in their attacks on air bases over the past week as they seek to counter the government's use of MiG warplanes and helicopter gunships.
In Idlib province, a major battleground on the Turkish border, rebels attacked an army roadblock in Harem district early Saturday, killing or wounding nine soldiers, the Observatory said.
State media reported a "terrorist" group had killed five members of a family in the central Marjeh district of Syria's second city Aleppo, a key battleground in the more than 17-month-long conflict.
In the central province of Hama, rebels killed at least four soldiers in a roadblock attack, the Observatory said.
It also said 18 unidentified bodies were found in the Damascus area on Saturday, most with their hands tied and some bearing signs of torture.
At least 38 people -- 13 civilians, 12 troops and 13 rebels -- were killed countrywide on Saturday, the Observatory said.
The official SANA news agency reported that 225 prisoners "implicated in recent events but without blood on their hands," were freed in Damascus province on Saturday.
In talks with Syrian Prime Minister Wael al-Halaqi in Tehran on Friday, UN chief Ban Ki-moon said Assad's regime must stop using its heavy weapons.
But Russia's top diplomat said 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," said Lavrov.
"Either people are naive or it is some sort of provocation," he added.
Lavrov stressed that Moscow, a Cold War era ally of Damascus, was not trying to support Assad or his government but was basing its policies on the daily situation on the ground.
"No matter your view of the Syrian regime, it is completely unrealistic in the current situation -- when there is fighting in the cities -- to say that the only way out is the unilateral capitulation of one of the opposing sides.
Veteran troubleshooter Lakhdar Brahimi has now taken up faltering international attempts to end the conflict but with low expectations that he will have any more success than his predecessor, former UN chief Kofi Annan.
Algeria's former foreign minister was to hold meetings Saturday at UN headquarters, a UN spokesman said.
Both Damascus and Aleppo have seen persistent fighting between troops and rebels in a conflict that has now claimed more than 26,000 lives since March 2011, according to Observatory figures.
The International Committee of the Red Cross has warned that the situation across large swathes of Syria is "edging towards irreversible deterioration."
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 Turkish Prime Minister Recep Tayyip Erdogan has since acknowledged that any such move would require UN backing and would be far too risky without the prior establishment of a no-fly zone.
Jordan's government said on Saturday that it needs $700 million in international aid to cope with an anticipated influx of 240,000 refugees from across the border in Syria.