Elsewhere, government forces bombed, shelled and rained rocket fire on cities across the country in the latest efforts by al-Assad's forces to drive back rebel advances, opposition activists said.
The airstrikes signal a sharp escalation in the fighting by forces loyal to al-Assad and rebels seeking his ouster, raising concerns among Syria's neighbors that the 21-month-old civil war could spill across the borders.
Neighboring countries have reported deadly border skirmishes with either Syrian forces or rebels.
In June, Syrian anti-aircraft defenses shot down a Turkish military reconnaissance jet, killing two pilots, after it briefly crossed into Syrian airspace in the eastern Mediterranean Sea. Months later, errant Syrian artillery shells hit the border town of Akcakale, killing five Turkish civilians.
As a result, Turkey has asked NATO for Patriot missiles to bolster its air defenses, a request NATO is expected to approve on Tuesday.
The United States, Germany and the Netherlands, which all have Patriot capabilities, have signaled they may be willing to contribute missiles should NATO approve the deployment to Turkey.
NATO Secretary General Anders Fogh Rasmussen stressed to CNN's Christiane Amanpour on Monday that the possible deployment of Patriot missiles is "purely defensive," and not aimed at preparing a no-fly zone.
"We have no intention to intervene militarily in Syria. We will do what it takes to protect our ally Turkey," he said.
However, Russia reiterated its opposition.
"We don't consider that this will lead to the improvement of security in the current situation." President Vladimir Putin's spokesman, Dimitri Pesvok, said Monday.