Syrian government forces have used helicopter gunships and tanks to bombard opposition strongholds across the country, including towns in the northern province of Idlib and in the capital Damascus, opposition sources have said. 

The attacks were reported on Friday after Mustafa al-Sheikh, a former brigadier general who defected from the Syrian army, said that about 2,500 Syrian troops and 170 tanks had assembled at an infantry school near the village of Musalmieh northeast of the city of Aleppo, just 30km from the Turkish border. 

The mass military deployment came after Turkey amassed troops and deployed surface-to-air missile launchers along its southern border with Syria in response to the shooting down of a Turkish warplane by Syrian forces.

"They're either preparing to move to the border to counter the Turkish deployment or attack the rebellious [Syrian] towns and villages in and around the border zone north of Aleppo," Sheikh told reporters. 

Omar Abdallah, an activist in Idlib co-ordinating with the Free Syrian Army, the main armed opposition group, said: "After taking hits in rural Aleppo and Idlib, the army is regrouping. There is speculation that these forces intend to ring Aleppo starting July 1."

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Rebel sources in Turkey's Hatay border region said Assad's helicopters attacked Saraqeb, an opposition-controlled town deep in Idlib province.

The new offensive was launched as government forces continued to shell residential areas in the Damascus suburb of Douma, where activists accuse the government of killing more than 50 people in just the last two days.

Activists released a video purportedly showing dozens of bodies of civilians, including women and children, who were killed for Syrian forces. 

Opposition groups say at least 190 people were killed by government forces on Thursday, one of the deadliest days of Syria's 16-month uprising.