Fierce fighting rocked northern Syria on Friday as Ankara pressed its call for safe havens to be set up in the country to stem the refugee exodus and protesters demanded the fall of the regime.
UN chief Ban Ki-moon told Syria's premier at a summit in Tehran that Damascus must stop using heavy weapons in the conflict, and the International Committee of the Red Cross warned of a fast deteriorating humanitarian situation.
Clashes erupted in the battleground city of Aleppo, less than 50 kilometres (30 miles) from the Turkish border, and rebels attacked Abu Zohur air base in Idlib province on the border where they said they shot down a MiG warplane on Thursday, a rights group said.
In some of the heaviest fighting, rebels clashed with army troops in Albu Kamal, a town in the eastern province of Deir Ezzor, said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
"The rebels have seized several army posts, including a base in the Hamdan military airport," Abdel Rahman told AFP.
Turkish Foreign Minister Ahmet Davutoglu expressed frustration at the reluctance of Ankara's Western allies to heed its calls for protected camps inside Syria to cope with the rapidly swelling numbers of fleeing civilians.
The United Nations estimates that in Aleppo alone at least 200,000 of the city's 2.7 million population have fled since it became a major battleground on July 20.
Rebels attacked a security service building in west Aleppo before dawn, and clashes erupted in the districts of Saif al-Dawla and Salaheddin in the southwest and Hanano in the northeast, the Observatory said.
In Idlib province, rebels seized part of the Abu Zohur base in heavy clashes, the Britain-based watchdog said.
The rebels say that aircraft from Abu Zohur have been used by President Bashar al-Assad's regime to launch devastating strikes on rebel-held areas.
The Observatory reported at least 70 people killed on Friday, with the overall death toll since the uprising against Assad's rule erupted in March last year now topping 26,000.
It warned that hundreds of families remain trapped in the central city of Homs as an army siege of rebel-held districts entered its 90th day on Friday.
"The injured and the elderly need medicine, the children need milk. But nobody in the world cares any more, no one at all," activist Abu Bilal told AFP in Beirut via Skype. "Here in Homs, we are all dying a slow death."
Protesters demonstrated in Damascus, Daraa, Hama and also in Aleppo, chanting anti-regime slogans.
"We will not surrender, despite your tanks and guns!" they shouted in Assali, a Damascus district, while chants like "Treacherous soldier, shame on you!" echoed in Daraa as protesters accused regime troops of killing civilians.
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-- 'Stop using heavy arms' --
The UN's Ban told Syrian Prime Minister Wael al-Halaqi and Foreign Minister Walid Muallem that the fighting must stop, "with the primary responsibility resting on the government to halt its use of heavy weapons."
He said at a news conference in Tehran: "What is important at this time is that all the parties must stop the violence. All those actors who may be providing arms to both sides... must stop."
The International Committee of the Red Cross warned that the situation across large swathes of Syria was "edging towards irreversible deterioration."
A Turkish diplomat told AFP Ankara would maintain its campaign for a safe haven inside Syria for the tens of thousands of civilians fleeing the violence after Davutoglu's appeal to the UN Security Council fell on deaf ears.
"We will continue to appeal to the international community to act," the diplomat said on condition of anonymity.
Davutoglu on Thursday told the Security Council to act "without delay" to set up safe havens, warning that 80,000 Syrians are already in camps in Turkey, with 4,000 crossing the border each day.
"How long are we going to sit and watch while an entire generation is being wiped out by random bombardment and deliberate mass targeting?" Davutoglu asked, slamming the Security Council's failure to act.
But world powers failed to reach agreement on his proposal which would imply authorising a highly controversial protective military operation.
Syria's Deputy Foreign Minister Faisal Muqdad told Iran's Arabic-language Al-Alam television: "Turkey nowadays trains and allows in terrorists, allows in Al-Qaeda. Most of the terrorists in Syria come from Turkey."
Syria ally Russia on Friday again called for an immediate end to the violence.
"The violence in Syria has to be ended immediately," and work begun on a "political settlement to end the suffering of the civilian population," a foreign ministry statement said.
In other developments, Canada expanded sanctions against Damascus, adding 50 names to a list of people and organisations linked to Assad's regime in Ottawa's 10th tightening of sanctions against Syria in the past year.
And the news agency MAP said Morocco will host the next "Friends of Syria" meeting in October, not September as originally scheduled.