The United Nations says fighting in the city of Aleppo of Aleppo has increased significantly in the past few days.
Sausan Ghosheh of the UN mission in Syria has told the BBC that opposition forces are now in possession of heavy weapons, including captured tanks.
More than 200,000 people have fled Aleppo in recent weeks, the UN says, as government forces battle to oust Free Syria Army (FSA) rebels from the country's biggest city.
The rebels appear to control large parts of the city despite government assertions that they have suffered heavy losses.
Ms Ghosheh says observers are reporting heavy exchanges of fire and, for the first time, firing from fighter aircraft.
The BBC's correspondent in neighbouring Lebanon says there are reports that the FSA may have been given shoulder-fired anti-aircraft missiles, which could reduce the threat posed by government helicopter gunships.
Execution of prisoners criticised
Meanwhile, the opposition Syrian National Council has criticised rebel gunmen for executing several prisoners who were said to belong to a pro-government militia in Aleppo, after footage of the incident emerged.
Human Rights Watch has said the incident could potentially be a war crime.
Video posted online shows the men, some of them bruised and bloodied, being put up against a wall half-naked and shot with Kalashnikov rifles.
One of the men killed has been identified as Ali Zeineddin al-Berri, known as Zeno, who is accused of leading a pro-regime militia group that killed 15 FSA fighters during a truce in Aleppo earlier this week.
Abdullah Omar, a cousin of a rebel fighter killed by the militia, says it's naive to imagine the rebels will have the same standards as a regular army.
"We have to remember of course that the FSA is predominantly made up of defectors from Syria's army," he says, "and it is absurd to imagine that merely by defecting they will magically transform themselves into an organisation that adheres by international standards of warfare."
