The war of words between Syria and Turkey escalated on Thursday when 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 after he said the jet had been carrying "equipment and ammunition shipped to the Syrian defence ministry" from a Russian military supplier.

France warned of the risks posed by the rising tensions between the two neighbours, which have exchanged fire over their border in recent days, amid fears that the Syrian civil war could ignite broader regional conflict.

Inside Syria, the fighting raged on, with least 87 Syrian soldiers reported killed over the day, according to the Syrian Observatory for Human Rights -- the heaviest military toll since the start of the conflict.

Syrian authorities challenged Erdogan to show the weapons he alleged had been seized from the plane, which was intercepted by Turkish fighters on Wednesday and forced to land for an inspection before being allowed to fly on.

"The Turkish prime minister continues to lie in order to justify his government's hostile attitude towards Syria," the Syrian foreign ministry said in a statement.

"The plane did not carry ammunition or military equipment and Erdogan's comments lack credibility and he must show the equipment and ammunition at least to his people," the information ministry said.

Meanwhile, rebels fighting forces loyal to Syrian president Bashar al-Assad won more territory as they bid to secure a "buffer zone" in a swathe of land abutting the Turkish border, an AFP reporter said.

And in Damascus, a powerful blast rocked the military justice building, the Observatory reported, in what state television said was a terrorist attack.

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

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

In the plane incident, Ankara scrambled two jets on Wednesday to force the Syrian Air airliner to land after receiving intelligence its cargo did not comply with civil aviation rules, Turkish officials said.

The aircraft with 35 passengers on board was grounded for nine hours before it was finally allowed to resume its journey to Damascus.

Damascus denounced the interception as "hostile and reprehensible... another sign of the hostile policies of the Erdogan government, which harbours (rebels) and bombs Syrian territory."

It demanded Turkey return the cargo seized at Ankara's Esenboga Airport.

"Turkish military aircraft... forced the plane to land without giving prior warning. The military aircraft were so close that there could have been an accident," said Syrian Air director Aida Abdel Latif.

Russia, Syria's ally and main arms supplier, also denounced Ankara.

"We are concerned that this emergency situation put at risk the lives and safety of passengers, who included 17 Russian citizens," said Russia's foreign ministry.

It denied the plane had been carrying weapons or military equipment.

Tension has been high between Ankara and Damascus since the conflict erupted in March 2011 and were inflamed after a series of shell strikes from Syria on Turkish soil, including one attack that killed five civilians last week.

-- Rebels seek buffer zone --

Rebels are seeking to secure a buffer zone in the northwestern province of Idlib, and this week won control of the strategic town of Maaret al-Numan on the highway linking Damascus to second city Aleppo.

An AFP correspondent in the town said the insurgents had completely cut off the highway Thursday, choking the flow of troops to battlefields in the north.

Fierce fighting raged on in the periphery of Maaret al-Numan, where rebels have surrounded the key military bases of Wadi Daif and Hamdiyeh used by troops to bombard the town.

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 before fleeing.

The floor of two cellars, which held soldiers suspected of trying to defect or supporting the rebellion, were covered in blood and stained clothing, AFP witnessed.

The Observatory's nationwide toll for the day was at least 210 deaths -- 87 soldiers, 64 rebels and 59 civilians -- adding to the overall tally of more than 32,000 killed in the nearly 19-month conflict.

"More than 70 percent of the deaths today are soldiers -- that shows the intensity of the fighting," said the Observatory's president, Rami Abdel Rahman.

French President Francois Hollande on Thursday warned of the risky situation between Turkey and Syria.

"It's a risk, and Turkey has been especially restrained," Hollande told French television and radio. "I welcome the attitude of its leaders because there have been aggressions and provocations."

In Damascus, a blast in the Mazzeh district targeted the military justice building, near the ministry of higher education, the Syrian Observatory said.

State television said "a bomb exploded near the ministry of higher education" and that two people were wounded in the "terrorist attack."

The Syrian capital has been rocked by a string of bomb blasts, including suicide bombings, mostly targeting government buildings, since the March 2011 outbreak of an anti-regime revolt.