Syrian mortar fire again struck a Turkish border village on Sunday, prompting artillery retaliation for the fourth day as fierce fighting rocked the key city of Aleppo and rebels lost ground in Damascus.

The Syrian mortar round struck hit Akcakale -- site of a similar strike on Wednesday -- as Turkish Foreign Minister Ahmet Davutoglu said President Bashar al-Assad should be replaced by Vice President Faruq al-Shara.

The 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.

A Syrian rebel takes part in a protest against the regime in the northern city of Aleppo on October 5. Aleppo was shaken on Sunday by the heaviest fighting of an almost three-month offensive against rebels in Syria's second city, residents said, as the insurgents lost ground in the capital Damascus.

On Wednesday five civilians were killed in Akcakale in a mortar strike that provoked counter-fire, in the most serious incident since Syrian anti-aircraft fire brought down a Turkish warplane in June.

That incident caused a spike in tensions between the former allies and renewed fears of a broader conflict.

Turkey's parliament on Thursday gave the government the green light to use military force against Syria if necessary.

Akcakale's mayor was quoted by the semi-official Anatolia news agency as saying Sunday's mortar hit prompted an immediate response by Turkish artillery.

"Thank God there were no victims. Turkish artillery immediately responded to the shots that came from Syria," Abdulhakim Ayhan said.

The UN Security Council on Thursday strongly condemned cross-border attacks by Syria and called for restraint between the neighbours.

Syria's commercial capital Aleppo, meanwhile, was rocked by the heaviest fighting of an almost three-month offensive against rebels, residents said.

Destruction in the Saif al-Dawla district of Syria's northern city of Aleppo, seen on October 6. AFP correspondents have passed through large swathes of territory in the Idlib and Aleppo provinces of northern Syria that have fallen outside government control, with residents managing their own affairs.

An AFP correspondent said warplanes were overflying the rebel-held Bab al-Hadid and Shaar neighbourhoods, 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.

"It looks like the army is trying to push the rebels as far as it can from the Hanano barracks," the resident said, asking not to be identified.

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.

Syrian rebels take position inside a building during clashes with government forces in the Saif al-Dawla district of the northern city of Aleppo on October 6. As the fighting raged in Aleppo, Syrian state television said that government forces had pushed rebels out of two of their strongholds in Damascus province, Qudsaya and Hameh, where a watchdog said the bodies of 10 men were found.

"Hameh and Qudsaya in Damascus province have been cleansed from the armed terrorists," the channel said, using the regime's blanket term for the rebels.

The Syrian Observatory for Human Rights said the government had taken control of Hameh and said 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.

A Syrian rebel commander addresses demonstrators during an anti-regime protest in the northern city of Aleppo on October 5. Rebels cemented their control of Syria's northern frontier with Turkey after fierce clashes with the army, as their bastions in other parts of the country came under heavy shelling on Sunday.

Since then, regime forces have pushed the rebels to the outskirts of the capital but have lost control of several border crossings and are battling to fully retake Aleppo.

The Observatory, which gave a toll of 50 people killed so far on Sunday, also reported that regime forces pounded the rebel-held town of Tal-Abyad in the northern province of Raqa, on the border with Turkey.

Turkey on Friday shelled a Syrian military position south of Tal-Abyad, as part of its retaliation for Wednesday's killings in Akcakale.

A picture taken on October 4, 2012 shows Turkish soldiers taking position in the southern border town of Akcakale. The Syrian mortar round struck hit Akcakale -- site of a similar strike on Wednesday -- as Turkish Foreign Minister Ahmet Davutoglu said President Bashar al-Assad should be replaced by Vice President Faruq al-Shara.

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 clashes at Khirbat al-Joz... ended when fighters of the rebel brigades took control of the area," said the Britain-based watchdog.

"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.

Nearly 80 percent of towns and villages along the border are now outside regime control, according to the Observatory.

With tensions between Turkey and Syria spiking, Davutoglu urged that Shara take the helm in Syria.

"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 last year.