Twin suicide bombings hit a Syrian air force compound near Damascus killing dozens of people on Tuesday, monitors said, as rebels took a key town on the road between the capital and second city Aleppo.
Turkey, meanwhile, again warned Syria 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 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.
On the increasingly bloody battlefield, state television said troops entered a rebel district of central city Homs and were "pursuing the remnants of the terrorists" -- the regime's term for rebels.
Pro-government media remained silent on Monday night's suicide attacks in Harasta, a town northeast of Damascus, but a security source said the assault had been largely foiled, although some people were hurt when one vehicle blew up.
The blasts were claimed by the jihadist Al-Nusra Front, 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 said "dozens of people" died in the bombings, and that the fate of "hundreds of prisoners" held in the building's basement was unknown.
"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," he told AFP.
AFP was unable to verify either of the widely differing accounts due to severe restrictions on journalists.
The 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 Aleppo, which has been fiercely contested since mid-July, and in Idlib province near the Turkish border.
Rebels took control of Maaret al-Numan, a strategic Idlib province town on the highway linking Damascus with Aleppo, after a fierce 48-hour gunbattle, the Observatory said.
"This is your tank, O Bashar!" a group of about 20 rebels shouted in a video posted by activists online as they fired off celebratory gunfire at a captured army checkpoint in the town.
The reported seizure came as soldiers moved into the central city of Homs, farther south on the same highway, in a bid to finish off insurgents there and free up forces for battle zones like Aleppo.
State television, meanwhile, said troops entered the rebel district of Khaldiyeh in the besieged city of Homs.
An activist confirmed the army had "stormed part of Khaldiyeh," but the Observatory said the neighbourhood remained in rebel hands, although fighting was intense.
"The catastrophe is that there are 800 families trapped in Homs. It will be an unprecedented massacre if they take over the district," said the activist who identified himself as Abu Bilal.
A security official told AFP on Monday the army hoped to eliminate the last pockets of resistance in Homs and nearby Qusayr by the week's end to free up troops for battle zones in the north, such as Aleppo.
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," he said.
Ban urged "the opposition forces to agree to this unilateral ceasefire when and if the Syrian government declares it," and appealed for countries to stop arming both sides.
Turkish Prime Minister Recep Tayyip Erdogan warned of retaliation against 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.
Erdogan spoke as his armed forces chief inspected troops on a tour of the heavily fortified border after a number of shells landed on Turkish soil, including one strike that killed five civilians last week.
The Observatory said violence across Syria killed at least 72 people on Tuesday. It said more than 32,000 people have died since the revolt against Assad erupted in March 2011.
The conflict has also driven tens of thousands of people from their homes, and the first formal camp inside Syria for the displaced began admitting families on Tuesday, just a stone's throw away from the border with Turkey.