Syria regime helicopter downed as Ankara threatens Damascus

Tensions escalated Tuesday between Syria's regime and rebel-backer Turkey as a Syrian military helicopter was shot down and Ankara warned of a "heavy price" for any attacks on its forces.

The new flare-up, a day after regime shelling killed five Turkish troops, came as government forces battling rebels in northwestern Syria took full control of a key highway linking the country's four largest cities.

The advance marked another step in President Bashar al-Assad's campaign to retake Syria's last rebel-held pocket, where nearly 700,000 civilians have fled violence since December in the largest exodus since the start of the war.

Shortly after the M5 motorway was recaptured, a rocket attack downed a Syrian regime helicopter in Idlib province, killing both pilots, an AFP correspondent and a war monitor said. Syrian state news agency SANA confirmed the downing of the aircraft and the killing of its crew, saying it was caused by a rocket fired from a part of Idlib where Turkey-backed rebels operate.

The Britain-based Syrian Observatory for Human Rights said the helicopter was hit by a rocket fired by Turkish forces, though Ankara did not claim responsibility. An AFP correspondent saw the bodies of the two pilots and the mangled remains of the helicopter at the site of the crash near the village of Qaminas, southeast of Idlib city.

Regime forces later shelled areas near a Turkish observation post in the same village, said the Observatory and an AFP correspondent who saw plumes of smoke rising following a large fiery blast.

The Observatory said the attack killed at least three people but it was unclear if they were Syrian rebels or Turkish troops.

Turkey, which has troops deployed in several locations in northern Syria, continues to support rebel groups battling the Assad regime or acting as proxies against Kurdish forces.

Along with regime ally Russia, it is the key foreign broker in northern Syria, but a 2018 deal aimed at averting a major offensive has failed to take hold.

On Tuesday, Erdogan said Turkish troops would continue to respond to Syrian regime attacks. "The more they attack... our soldiers, they will pay a very, very heavy price," he told a televised ceremony in Ankara. Erdogan said he would reveal his next steps on Wednesday. The Syrian army hit back in a later statement saying it was "ready to respond to the aggressions of the occupying Turkish army", accusing Ankara of targeting its positions with rockets. US Secretary of State Mike Pompeo said Washington would work with Turkey on its response to Monday's attack. "The ongoing assaults by the Assad regime and Russia must stop," Pompeo said on Twitter.