Twin attacks by suicide bombers on a Syrian air force compound near Damascus killed dozens of people, a watchdog said on Tuesday, adding that it feared for the safety of hundreds of prisoners being held there.
Turkey, meanwhile, again warned Syria that it would not hesitate to retaliate for any strike on its soil as the country's top military commander visited troops stationed along the reinforced border.
And with fighting spilling across into both Turkey and Lebanon, UN chief Ban Ki-moon urged President Bashar al-Assad's regime to declare a unilateral truce while NATO head Anders Fogh Rasmussen urged restraint by all parties.
Elsewhere on Syria's increasingly violent battlefield, state television said troops entered the rebel district of Khaldiyeh in the central city of Homs and were "pursuing the remnants of the terrorists" -- the regime's term for rebels.
There was no official comment on Monday night's suicide attacks in the town of Harasta, northeast of the capital, but a security source told AFP the assault had been largely foiled, although several people were hurt when one vehicle blew up.
The attacks were claimed by the Al-Nusra Front jihadist group, which said one attacker drove a booby-trapped car and a second an explosives-packed ambulance.
Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP that "dozens of people" died in the twin suicide bombings.
"The fate of hundreds of prisoners being held in the basements of the (building) is still unknown," added Abdel Rahman.
"The regime has not said a word about what happened last night. I hold the regime responsible for the fate of the prisoners. They shouldn't be holding all of these people in the first place."
Due to severe restrictions on journalists operating on the ground in Syria, AFP was not able to verify either of the widely differing accounts.
The Britain-based Observatory said the attacks sparked intense fighting in Harasta between rebels and the army, which at daybreak pounded the town with shells.
It said Syrian forces on Tuesday also rained shells down on rebel belts in the second city of Aleppo, which has been fiercely contested since mid-July, and in Idlib province near the Turkish border.
-- Fighting rages for rebel stronghold in Homs --
State television, meanwhile, said Syrian troops entered the rebel district of Khaldiyeh in the besieged central city of Homs on Tuesday.
But the Observatory said that while fighting was raging near the neighbourhood, it remained in rebel hands.
An activist in the embattled city confirmed that the army had "stormed part of Khaldiyeh."
"The catastrophe is that there are 800 families trapped in Homs. It will be an unprecedented massacre if they take over the district," Abu Bilal told AFP via Skype.
A security official told AFP on Monday that the army hoped to eliminate the last pockets of resistance in Homs and the nearby town of Qusayr by the end of the week to free up troops for battle zones in the north, such as Aleppo.
In Paris, UN chief Ban urged a unilateral truce by Assad's regime.
"I have conveyed to the Syrian government (a) strong message that they should immediately declare a unilateral ceasefire," said Ban, addressing a joint news conference with French President Francois Hollande.
Ban urged "the opposition forces to agree to this unilateral ceasefire when and if the Syrian government declares it," and called on countries supplying arms to both sides to stop in order to ease the suffering of the Syrian people.
In Ankara, Turkish Prime Minister Recep Tayyip Erdogan warned of retaliation to Syria's "aggressive position."
"It has become inevitable for our armed forces to retaliate in kind... as the Syrian administration maintains its aggressive position," he told lawmakers.
He spoke as Turkish armed forces chief of staff General Necdet Ozel was inspecting troops on a tour of the heavily fortified border region after a number of shells landed on Turkish soil, including one strike that killed five civilians last week.
After the deadly shelling in Akcakale village on Wednesday, Turkey's parliament gave the government the green light to use military force if necessary against one-tie ally Syria.
Turkey bombarded Syrian army positions again on Monday after a shell hit another border town.
"Troops loyal to the Assad administration are shelling our territory... We are retaliating in kind. We are currently doing whatever the rules of engagement require us to do," said Erdogan, who has been one of the most vocal opponents of Assad's regime.
NATO head Anders Fogh Rasmussen warned in Brussels against the dangers of the conflict in Syria escalating, saying alliance member Turkey had shown commendable restraint in response to shelling of its border area.
"Obviously Turkey has a right to defend herself within international law," Rasmussen said, noting that the alliance too has "all necessary plans in place to protect and to defend Turkey if necessary."
The Observatory says more than 32,000 people, mostly civilians, have died since the revolt against Assad erupted in March last year.