Syria's army launched multiple attacks on rebel positions on Tuesday in the northern province of Aleppo, including insurgent-held areas of a key airbase, a monitoring group said.
Syrian government forces shelled parts of the Minnigh military airbase a day after rebels seized its strategic radar tower, months into a battle for the site, the Syrian Observatory for Human Rights said.
"Parts of Minnigh military airbase were shelled by regime forces... Rebels are in control of large swathes of the airbase," the Britain-based Observatory said.
A military source told AFP on condition of anonymity that heavy clashes were raging for a third day at the base, but denied that any part of the airport was under rebel control.
He said the fighting was not part of a broader campaign the regime has pledged to launch to re-take the city of Aleppo, large parts of which are in rebel hands.
Other areas of Aleppo came under fire, two days after pro-regime media said an army campaign in Aleppo province would began within "hours or days."
Regime forces shelled the opposition-controlled villages of Deir Hafer and Al-Bab, and hit the insurgent stronghold of Marea with rockets, the Observatory said.
There were unconfirmed reports of a ground-to-ground missile strike in northern Aleppo province, the Observatory added.
The violence came almost a week after troops loyal to President Bashar al-Assad, aided by fighters from Lebanon's Hezbollah, seized the strategic town of Qusayr from the rebels.
Armed with a new confidence and boosted by its Lebanese Shiite ally's help in Qusayr, Assad's regime will now look to taking strategic rebel positions elsewhere in the country, activists say.
"The regime's aim is to attack the northern and western countryside of Aleppo, in order to cut off supplies to rebels elsewhere in the province," said Aleppo-based activist Mohammad al-Khatib, who spoke to AFP over the Internet.
"The regime is indeed preparing for something. There have been signals of this for the past month," he added, though he dismissed the army's capacity to advance in Aleppo.
"The regime's announcement of a campaign is part of the media war," Khatib said.
Elsewhere in Syria, the army shelled rebel positions in the southern province of Daraa, where insurgents have also made gains in recent weeks, said the Observatory, which relies on a network of activists, doctors and lawyers for its reports.
Among the army's targets were districts of Daraa city, which activists refer to as the "cradle of the revolution" against Assad, as it saw some of the first anti-regime demonstrations in March 2011.