DAMASCUS: Syria accused Ankara of “flagrant aggression” Sunday after Turkish forces shot down a warplane near the border, raising tensions as Syrian loyalists and rebels battle to control a frontier crossing.

Relations between the neighbouring states have collapsed during the Syrian conflict with Ankara squarely backing rebels trying to topple President Bashar al-Assad.

It was the most serious incident since Turkish warplanes last September downed a Syrian helicopter that Ankara said was two kilometres (more than a mile) inside its airspace.

A Syrian military source said Sunday Turkey shot down the warplane “in a flagrant act of aggression that is evidence of (Turkish Prime Minister Recep Tayyip) Erdogan's support for terrorist groups.”

The aircraft “was chasing terrorist groups inside Syrian territory at Kasab,” said the source, referring to the disputed border crossing. The pilot was able to eject.

Syrian troops and rebels have been locked in deadly fighting since Friday for control of Kasab in the northern province of Latakia, a regime stronghold.

The battle erupted after three jihadist groups, including al Qaeda's Syria affiliate Al-Nusra Front, announced the launch of an offensive Tuesday in Latakia dubbed “Anfal,” or “spoils of war.”

In a move expected to further exacerbate tensions, Erdogan praised Turkey's military for downing the warplane and warned Syria against any response.

“I congratulate the chief of general staff, the armed forces and those honourable pilots... I congratulate our air forces,” said Erdogan.

“Our response will be heavy if you violate our airspace,” he warned Damascus.

Turkish media reported that the military told two Syrian jets approaching the border that they should turn away, but scrambled F-16 jets when one ignored the warning.

The Britain-based Syrian Observatory for Human Rights said the plane was bombing rebels fighting to seize Kasab when it was hit, catching fire and crashing inside Syrian territory.

The monitor said jihadists had overrun the crossing but were still battling loyalist forces in the area and in nearby Kasab town. It also said the government has sent military reinforcements.

The battle for Kasab erupted on Friday and at least 80 fighters on both sides have been killed.

Rules of engagement

Turkey toughened its rules of engagement after one of its fighter jets was downed by the Syrian air force in June 2012.

It now says that any military approach towards the border from Syria would be considered a threat.

Sunday's action prompted an angry response from the foreign ministry in Damascus which, echoing the military source, accused Turkey of “interference” in Kasab and siding with the rebels.

Turkey's “unjustified military aggression against Syrian sovereignty in the Kasab border region over the past two days proves its implication in the events in Syria,” it said in a statement.

According to the Observatory, which relies on a network of activists and medics on the ground for its reports, the fighting in Latakia spread Saturday to a string of villages under regime control.

As a result, “significant military reinforcements have been sent to the government forces,” it said.

Latakia province, which includes Assad's family village, is considered a regime stronghold, and many of its residents are from his Alawite minority.

Large parts of the province have remained relatively insulated from the three-year conflict that has reportedly killed more than 146,000 people.

Turkey is a staunch opponent of the Assad government and hosts more than 750,000 refugees, many in camps along the border.

Ankara supports the rebellion against Assad's regime but also worries that jihadists fighting in Syria could threaten its own security.

On Friday, Ankara warned that it would retaliate “in kind” if jihadists in the northern province of Aleppo attack the historic Tomb of Suleyman Shah, which is inside Syria but under Turkish jurisdiction.

Also on Sunday a gunman was killed and 13 wounded in a firefight between pro- and anti-Damascus factions in Beirut, the latest spillover from the Syria war into Lebanon.