A Syrian opposition official has asked for no-fly zones across Syria and safe havens patrolled by foreign forces near the borders with Jordan and Turkey.
Abdel Basset Sida, head of the Syrian National Council, said the United States had realised that the absence of a no-fly zone to counter President Basher al-Assad's air superiority has hindered rebel movements in the country.
"There are areas that are being liberated," Sida told Reuters news agency by telephone from Istanbul on Sunday. "But the problem is the aircraft, in addition to the artillery bombardment, causing killing, destruction."
He said the establishment of secure areas on the borders with Jordan and Turkey "was an essential thing that would confirm to the regime that its power is diminishing bit by bit".
He was speaking a day after the US Secretary of State Hillary Clinton said her country and Turkey would study a range of possible measures to help Assad's foes, including a no-fly zone, although she indicated no decisions were necessarily imminent.
"It is one thing to talk about all kinds of potential actions, but you cannot make reasoned decisions without doing intense analysis and operational planning," she said after meeting Turkish Foreign Minister Ahmet Davutoglu in Istanbul.
A no-fly zone imposed by NATO and Arab allies helped Libyan rebels overthrow Muammar Gaddafi last year. The West has shown little appetite for repeating any Libya-style action in Syria, and Russia and China strongly oppose any such intervention.
On Sunday, Syrian troops stepped up their attacks on rebel-held areas across the country, as state media reported the death of one of its reporters, blaming "terrorists".
Assad's forces reportedly shelled the districts of al-Shaar and Hananu in the east of the city of Aleppo, while clashes were reported in the northern city's Salaheddin neighbourhood.
Syrian civilians desperate to check on their homes pushed into fluid front lines around Salaheddine, even as sniper fire cracked out and rebels warned them to stay away.
In the Damascus Suburbs province, machine gun fire was heard on Sunday in the town of al-Tel, where 15 civilians were killed in shelling and clashes a day earlier as troops tried to regain control from rebels.
Among those killed in government shelling in al-Tel is Yusuf al-Bushi, a Syrian army defector and a citizen journalist who worked with several international news organisations.
Journalists have suffered a number of casualties in the 17-month-old uprising against Assad, and in recent months there have been several attacks on pro-government media.
Elsewhere, government forces pursued a massive-scale incursion into the restive district of al-Shamas in the central city of Homs on Saturday, activists said.
The activists said that Assad loyalists had carried out summary executions in Shamas, a claim that could not be independently verified.
Rebel-held neighbourhoods in Homs have been under siege for more than two months, as shelling by government forces continues, while clashes with rebels at the edges rage.