Syrian rebels on offensive: watchdog

At least 87 soldiers have been killed by rebels in the Syrian army's worst day, officials say. (AAP)

At least 87 soldiers have been killed by rebels in the Syrian army's worst day, officials say. (AAP)

An observer group says 106 regime soldiers have been killed in two days as the Syrian rebels launched an offensive to control a key highway.

Syrian rebels have gone on the offensive, killing more than 100 soldiers in two days, a watchdog says, as tension between Syria and Turkey escalated over cargo seized from a Syrian Air jet.

Fourteen soldiers died in an attack on an army post in the southern province of Daraa on Friday, the Syrian Observatory for Human Rights said, a day after the army suffered 92 losses, the highest daily total for the military of the 19-month conflict.

As fighting raged on the ground, including in the northern provinces of Idlib and Aleppo, a war of words between Syria and Turkey grew angrier after Ankara said it had found military supplies on a passenger plane it intercepted en route between Moscow and Damascus.

The Syrian foreign ministry accused Turkish Prime Minister Recep Erdogan of lying when he said the jet had been carrying "equipment and ammunition shipped to the Syrian defence ministry" from a Russian military supplier.

Turkey's allies have warned of the risks embedded in the conflict between the neighbours, which have exchanged fire over their border in recent days, amid fears that the Syrian civil war could set off a regional conflagration.

Amid the growing alarm, German Foreign Minister Guido Westerwelle said on Friday he has hastily scheduled a visit to NATO partner Turkey.

Westerwelle, who is on a trip to China, said in a statement he would hold talks on Saturday with his Turkish counterpart Ahmet Davutoglu in Istanbul on "the situation in Syria and on the Turkish-Syrian border".

"The Syria situation has escalated. That fills us with the greatest concern," he said. "It is important that no one pours oil on the fire. We are counting on moderation and de-escalation."

The Britain-based Observatory said Thursday had marked one of the deadliest days of fighting since an anti-regime revolt erupted in March last year, with at least 240 people killed across the country, including the 92 soldiers, 67 rebel fighters and 81 civilians.

Of the soldiers killed on Thursday, 36 died in fighting in Idlib province, where much of the fiercest clashes have taken place over the past three months, it said.

In violence on Friday, regime war planes attacked two buildings in the Idlib town of Maaret al-Numan, where intense fighting has raged since rebels overran it on Tuesday after a fierce 48-hour gunbattle, the watchdog said.

An AFP reporter said that the rebels, by gaining control of a stretch of highway near Maaret al-Numan, were on Thursday able to cut off the route linking Damascus to Aleppo, choking the flow of troops to battlefields in the north.

Rebel spokesman Firaz Abdel Hadi said almost 300 people had been killed in three days in Maaret al-Numan.

Other sources said regime forces executed 65 prisoners before retreating. One survivor said guards had opened fire on 80 inmates and then fled.

In Aleppo province, rebels on Friday attacked a large air force post on the highway connecting Aleppo to Raqa province, further to the east, near to the Kweris military airport, according to the Observatory.

"The rebels attacked the air force battalion after midnight and the clashes went on until dawn, but the rebels definitely did not gain control of the post," observatory director Rami Abdel Rahman told AFP by phone.

Rebels suffered a number of casualties, but immediate figures were not available.

The rebels also suffered six losses in Friday's attack on the army checkpoint at Khirba in Daraa province, the watchdog said.

On his second regional tour, UN-Arab League peace envoy Lakhdar Brahimi meanwhile, has held talks with officials in Saudi Arabia, which like Turkey has called for Assad to quit and supports the rebels.

Saudi deputy foreign minister Prince Abdul Aziz bin Abdullah urged Brahimi to work for "an immediate halt to the bloodshed of the Syrian people" at a meeting in the Red Sea city of Jeddah, the Saudi news agency SPA reported.