A car bomb rocked police headquarters in the capital on Sunday after Syrian mortar fire again struck a Turkish border village, prompting fresh retaliation, and as fierce fighting swept the key city of Aleppo.
State news agency SANA said a bomb exploded in a vehicle in the car park of the police headquarters on Khaled bin al-Walid Avenue in central Damascus, killing a policeman and damaging the building.
Witnesses told AFP the blast was followed by heavy gunfire, while the Syrian Observatory for Human Rights reported casualties without giving a breakdown for the latest in a string of bombings of high-level security targets in Damascus.
Jihadist groups claimed responsibility for a September 26 bomb attack on armed forces headquarters in Damascus that the authorities said killed four guards.
Elsewhere, Syrian mortar fire again struck a Turkish border village, prompting artillery retaliation from Turkey, in what have become daily such incidents since Wednesday night.
The Syrian mortar round struck hit Akcakale -- site of a similar strike on Wednesday that killed five civilians.
The latest mortar round hit the grounds of a public building without causing casualties, Turkey's NTV news channel reported, adding that the building had been evacuated beforehand.
Turkey's parliament on Thursday gave the government the green light to use military force against Syria if necessary.
Akcakale's mayor Abdulhakim Ayhan was quoted by Turkey's semi-official Anatolia news agency as saying that Sunday's mortar firing prompted an immediate response by Turkish artillery.
The UN Security Council has strongly condemned cross-border attacks by Syria and called for restraint between the two neighbours whose ties have nosedived, with Ankara supporting the rebels fighting President Bashar al-Assad's regime.
Syria's commercial capital Aleppo, meanwhile, was rocked on Sunday by the heaviest fighting of an almost three-month offensive against rebels, residents said.
An AFP correspondent said warplanes were overflying the rebel-held Bab al-Hadid and Shaar neighbourhoods of the northern city, where witnesses reported fierce fighting.
"This is the worst fighting we've seen here since the beginning of the Aleppo war," one Bab al-Hadid resident told AFP.
"From early morning... there has been shelling on the area and clashes between the rebels in Bab al-Hadid and the army at the beginning of Arkoub district," near the Hanano military barracks.
A commander of the rebel Free Syrian Army said his fighters had expelled soldiers from the barracks in fighting on Sunday, but there was no independent confirmation of the claim.
-- State TV reports advances in Damascus --
As fighting raged in Aleppo, state television said government forces had pushed rebels out of two of their strongholds in Damascus province, Qudsaya and Hameh.
The Britain-based Syrian Observatory, which collates information from activists and medics on the ground, said the government had taken control of Hameh and that the bodies of 21 men were found there.
On July 18, rebels carried out a massive bombing in Damascus, killing Assad's brother-in-law, the defence minister and a general.
Since then, regime forces have pushed the rebels to the outskirts of the capital but have lost control of several border crossings in the north and are battling to fully retake Aleppo in a drawn-out battle.
The Observatory, which gave an updated toll of 107 people killed on Sunday, including 53 civilians, also reported that regime forces pounded the rebel-held town of Tal-Abyad in the northern province of Raqa, on the border with Turkey.
The Turkish military on Friday shelled a Syrian army position south of Tal-Abyad, as part of its retaliation for the killings in Akcakale.
On Saturday, rebels cemented their control of Syria's northern frontier after seizing the town of Khirbat al-Joz in the northwest province of Idlib after a pitched battle with regime troops, the Observatory said.
"The fighting lasted more than 12 hours and resulted in at least 40 dead among the regular forces, including five officers, and nine (rebel) fighters," it added.
With tensions between Turkey and Syria spiking, Turkish Foreign Minister Ahmet Davutoglu urged that Vice President Faruq al-Shara take the helm in Syria in place of Assad.
"Faruq al-Shara is a man of reason and conscience and he has not taken part in the massacres in Syria. Nobody knows the system better than he," Davutoglu said on public television channel TRT.
He stressed that the Syrian opposition "is inclined to accept Shara" as a future leader.
Shara, the most visible Sunni Muslim figure in the minority Alawite-led government, is trusted by the regime and was foreign minister for 15 years before becoming vice president in 2006.
Jordan's King Abdullah II, meanwhile, said it was "necessary to reach a political solution" to the Syria bloodshed which the Observatory says has so far killed more than 31,000 people since March 2011.