As many as 145 people have been reportedly killed by the fire of Syrian forces across the country on Sunday, Al Arabiya reported citing Syrian activists.
Two successive explosions rocked the Syrian capital Damascus earlier Sunday, the Syrian Media Center and the state television reported.
The first blast was heard near police headquarters in the Syrian capital. The state TV said it was caused by a “terrorist” car bomb.
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 Sept. 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 U.N. Security Council has strongly condemned cross-border attacks by Syria and called for restraint between the two neighbors 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 neighborhoods 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.
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 defense 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 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.