Turkish anti-aircraft fire shot down a Syrian warplane on Sunday as it bombed rebels fighting to seize control of a border post in northwestern Syria.

"Turkish air defences targeted a Syrian fighter bomber as it struck areas of the northern province of Latakia. The plane caught fire and crashed in Syrian territory," the Syrian Observatory for Human Rights said.

"A Syrian plane violated our airspace," Prime Minister Tayyip Erdogan told an election rally of his supporters in northwest Turkey soon after. "Our F-16s took off and hit this plane. Why? Because if you violate my airspace, our slap after this will be hard."

There was no immediate confirmation of the report from Syria, but the foreign ministry in Damascus issued a protest over Turkish "interference" in the Kasab region of Latakia.

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.

The Observatory said government forces and rebels on the ground were battling for control of a border post with Turkey in Latakia province.

"Fighters of Al-Nusra Front and other groups have overrun the crossing point and driven out government forces" and their allies, said Observatory head Rami Abdel Rahman.

But fierce clashes continued around the crossing point and in the nearby town of Kasab, in a battle which erupted on Friday.

"Significant military reinforcements have been sent to the government forces," said the Britain-based Observatory, which relies on a network of activists and medical sources on the ground for its reports.

The Observatory said earlier that nearly 80 fighters on both sides have been killed in the Kasab battle.

A Syrian security source said the rebels who had infiltrated from Turkey into Kasab suffered "heavy blows" on Sunday and denied reports that the army had lost control of its "Observatory 45" military post.

On Tuesday, Al-Nusra and Islamist groups Sham al-Islam and Ansar al-Sham announced the launch of an offensive dubbed "Anfal" in Latakia province.

The province, which includes President Bashar al-Assad's family village, is considered a regime stronghold, and many residents are from his Alawite minority.

And here's a video of a Turkish TV broadcast that saw the attack as it happened: