The Syrian army suffered its highest death toll since the start of rebel hostilities with at least 87 soldiers killed on Thursday, an observer group said, amid fears the civil war could become a broader conflict as tensions with Turkey escalate.
It was one of the deadliest days of fighting with at least 210 people killed across the country, including 64 rebel fighters and 59 civilians, according to the Syrian Observatory for Human Rights.
The war of words between Syria and Turkey heightened when Ankara said on Thursday that 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, alongside growing fears that the Syrian civil war could ignite broader regional conflict.
"It's a risk, and Turkey has been especially restrained," President Francois Hollande told French television and radio.
"I welcome the attitude of its leaders because there have been aggressions and provocations."
Of the soldiers killed on Thursday, 36 died in fighting in the northeast province of Idlib, where much of the fiercest clashes have taken place over the last three months.
Syrian authorities have challenged Erdogan to show the weapons he alleged were 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.
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.
In Damascus, a powerful blast rocked the military justice building, the Observatory reported, in what state television said was a terrorist attack.
And UN-Arab League peace envoy Lakhdar Brahimi 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 deployed two jets on Wednesday to force the Syrian Air liner 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."
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.
Tensions have been running high between Ankara and Damascus since the conflict erupted in March 2011.
They were further inflamed after a series of shell strikes from Syria on Turkish soil, including an attack that killed five civilians last week.