Syrian rebels have shelled an airbase used by regime forces to pound the northern city of Aleppo, with a human rights watchdog reporting the killing of dozens near Damascus in a series of government raids.
"Menagh military airport was bombarded on Thursday morning by a tank captured previously by the rebels," the Syrian Observatory for Human Rights said of the base 30km northwest of Aleppo, the country's commercial capital.
"We hit the airport using a tank that we captured from the Assad army. We attacked the airport a few times but we have decided to retreat at this time," a rebel fighter named Abu Ali told the Reuters news agency.
The UN confirmed on Wednesday that rebels battling President Bashar al-Assad's regime now had heavy armour, and that its military observers had seen the Syrian military use a fighter jet to attack rebels in Aleppo.
Intense explosions were reported along Syria's border with Turkey as the fighting raged.
The blasts have come from near the rebel-held town of Azaz, across the border from the Turkish town of Kilis. The area is a key route for refugees and rebels heading to fight in Aleppo, some 50km away.
The pro-opposition Syrian Observatory said government forces at the airbase had used artillery and rocket launchers to bombard the town of Tel Rifaat, which lies between the base and Aleppo.
Assad's troops, meanwhile, pounded the strategic Salaheddin district in Aleppo itself with tank and artillery fire while rebels tried to consolidate their hold on areas they have seized.
Heavily armed government troops are trying to drive a force of a few thousand rebel fighters from the city in a battle whose outcome could be a turning point in the conflict.
Mobile phone connections have been cut since Wednesday, leading to speculation among residents that an increase in military action might be imminent.
In Damascus, troops overran a suburb on Wednesday and killed at least 35 people, mostly unarmed civilians, residents and activist organisations said on Thursday."We will go after them in the whole of Aleppo, until the city is liberated," he said.
The Free Syrian Army's military chief in Aleppo, Colonel Abdel Jabbar al-Oqaidi, said the rebels had "thousands" of fighters in Aleppo.The rebels are consolidating areas they control in Aleppo, attacking police posts and minor military installations with some success. They claim to have seized three police stations this week.
"When the streets were clear we found the bodies of at least 35 men," a resident, who gave his name as Fares, said by phone from Jdeidet Artuz, southwest of Damascus.
"Regime forces entered the Jdeidet Artuz district on Wednesday and arrested around 100 young people who were taken to a school and tortured," it said in a statement.
Pro-regime newspaper Al-Watan on Thursday said security forces were "hunting down terrorist groups" in Damascus province.
On Wednesday, video emerged which appeared to show rebels executing regime loyalists in the embattled city of Aleppo.
The men, allegedly members of the Shabiha, or armed groups who have assisted in the government's crackdown, are lined up and shot at point blank range.
US 'secret order'
In a shift toward increased foreign involvement in the war, US President Barack Obama was reported to have signed a covert document authorising US support for the rebels.
World powers have watched with mounting concern as diplomatic efforts to find a negotiated solution have faltered and violence that has already claimed more than 15,000 lives.
The UN General Assembly will on Friday vote on an Arab-drafted largely symbolic resolution calling on Assad to stand down.
British Prime Minister David Cameron and Russian President Vladimir Putin were set for tense talks on the Syrian conflict on Thursday, before watching judo together at the London Olympics.
Britain has strongly criticised Moscow's refusal to back UN action against the Syria regime.
In-depth coverage of escalating violence across Syria
The Kremlin said on Wednesday that Putin would defend Russia's position on the crisis, on what is his first visit to Britain since 2005.
Amid growing signs that the humanitarian crisis in Aleppo is getting worse, the World Food Programme said it was sending emergency food supplies for up 28,000 people.
"The humanitarian situation is deteriorating in Aleppo and food needs are growing rapidly," the World Food Programme said.
It said three million Syrians need food, crops and livestock assistance, citing a survey by the United Nations and the Syrian government.
The FAO said that figure included 1.5 million Syrians who "need urgent and immediate food assistance over the next three to six months, especially in areas that have seen the greatest conflict and population displacement".
The UN says that some 200,000 of the city's estimated 2.7 million population have fled their homes, many of them taking refuge in schools and other public buildings.