Russia threatened on Thursday to respond with force if the United States bombs Syrian military positions where Russian servicemen are present, in a further escalation between Moscow and Washington over Syria.

Russian servicemen will have almost no time to try to find out the nationality of their attackers, Russian Defence Ministry spokesman Igor Konashenkov said in comments carried by state news agency TASS.

Russia's bases in Syria at Khmeimim and Tartus are equipped with S-400 and S-300 anti-aircraft missile systems, Konashenkov added.

The Syrian military has S-200 and Buk anti-aircraft missile systems, he said.

Russia is a key ally of the regime of Syrian President Bashar al-Assad. 

Moscow also announced on Thursday it was sending three missile-carrying warships to join its naval task force in the eastern Mediterranean in recent days.

Earlier this week, the US suspended its cooperation with Russia on Syria, citing the failure of a ceasefire and Moscow's inability to end the violence in the war-torn country.

The US supports some rebel groups seeking to overthrow al-Assad's regime.

Last month, US-led coalition jets bombed Syrian government positions near the eastern city of Deir al-Zour, killing 62 troops. 

The US Central Command said after the September 17 strike that it had no intention of targeting Syrian government forces.

The bombardment came two days before a truce brokered by the US and Russia, fell apart in Syria. 

Later, al-Assad's forces, backed by Russian airstrikes, launched a major offensive to dislodge rebels from the divided city of Aleppo in northern Syria.

Al-Assad vowed on Thursday to regain eastern Aleppo from the opposition.

In an interview with TV2 Denmark television, al-Assad said that his forces would continue to fight rebels until they leave Aleppo. 

"They have to. There's no other option. We won't accept that terrorists will take control of any part of Syria, not only Aleppo. This is our mission, and this is our goal, and this is our next step," he said.

Some 250,000 to 300,000 people are thought to remain in eastern Aleppo, which has been hit by devastating airstrikes and artillery shelling, including the repeated bombardment of its few remaining hospitals in recent weeks.

Late Thursday, opposition Syrian rebels shelled regime controlled areas in Western Aleppo, killing at 11 people and wounding 65  others, the Britain-based Syrian Observatory for Human Rights.     

Shells rained down on Jamileh neighbourhood and others areas in Suleimaniyeh in the western side of Aleppo, the observatory said.

Aleppo has been divided between a government-controlled western sector and the opposition-held east since a rebel offensive in mid-2012.

Earlier, regime forces recaptured from rebels half of the district of Bustan al-Basha in the northern sector of opposition-controlled eastern Aleppo, the observatory said.

"It is a major advance for the regime since the rebels took the area in 2013," the watchdog's head, Rami Abdel-Rahman, told dpa.

Syrian state television meanwhile said that government forces had recaptured key buildings in the area and pushed back "terrorists," a reference to rebels fighting to oust al-Assad.

On the Syrian-Turkish border, at least 29 people, mostly Syrian rebels, were killed in an explosion at Syria's rebel-controlled Atmeh border crossing, the observatory said.

The watchdog added that the explosion occurred when a man detonated his explosive belt among a crowd of Turkish-backed Syrian opposition fighters.

Atmeh, some 40 kilometres west of Aleppo, is a vital crossing point used for delivering aid and supplies to rebel-held areas in the north of Syria. 

The Islamic State extremist group claimed responsibility for the attack.

Amaaq News Agency, the militants' mouthpiece, reported that the attack had been carried by a suicide car bombing that targeted opposition rebels at the crossing.

According to Amaaq, around 80 people were killed or injured in what it called a "martyrdom operation," a term used by jihadists to refer to suicide attacks.

In August, a suicide bombing at the same crossing, claimed by Islamic State, killed some 32 Syrian rebels.

Turkey invaded northern Syria in August to attack Islamic State and Kurdish militants, though in recent weeks it has turned its focus on the jihadist group after the US urged Ankara, its NATO ally, to refrain from further attacks on the Kurds. 

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