Turkey said that UN agreement was necessary to set up safe havens in Syria to stem an exodus of refugees as fierce fighting raged in the north and protesters demanded the end of the regime.
UN chief Ban Ki-moon meanwhile told Syria's premier at a summit in Tehran that Damascus must stop using heavy weapons, and the International Committee of the Red Cross warned of a fast deteriorating humanitarian situation.
In the battleground city of Aleppo, less than 50 kilometres (30 miles) from the Turkish border, clashes erupted while 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 Friday's 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.
As the fighting raged, Turkish Prime Minister Recep Tayyip Erdogan told Kanal Turk television that no steps could be taken to create safe zones to protect refugees inside Syria "without any resolution at the UN Security Council".
His comments came the day after his 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.
But world powers failed to reach agreement on Davutoglu's proposal, which would likely involve a highly controversial protective military operation.
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, despite the appeal to the UN Security Council falling on deaf ears.
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."
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 Friday, 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. "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.
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".
Syria ally Russia 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.
The United States urged Syria to confirm it is detaining American freelance journalist Austin Tice, who has been missing in the country for more than two weeks.
And the news agency MAP said Morocco will host the next "Friends of Syria" meeting in October, not September as originally scheduled.