BEIRUT — Government forces battling rebels in northwestern Syria took full control on Tuesday of a key highway, over which they lost control in 2012, linking the country's four largest cities.

The latest gain marked another step in the government's advance against Syria's last rebel-held pocket, where intense fighting has displaced 700,000 civilians since December.

The remaining rebel fighters and some three million civilians are being pushed ever closer to the Turkish border by the regime's inexorable northward push.

The M5 motorway which links the capital Damascus to the second city of Aleppo through the cities of Homs and Hama has been a key government target for years.

On Tuesday, government and allied forces retook the Rashideen Al-Rabea area near Aleppo, the Syrian Observatory for Human Rights said.

"That means they control the entire M5 for the first time since 2012," Rami Abdel Rahman, the head of the UK-based war monitor, told AFP.

The government's advance also helps secure Aleppo, the country's former industrial hub, which still comes under sporadic rocket fire from holdout rebel groups.

The war in Syria, which started with a popular movement against President Bashar Al-Assad's rule that was brutally repressed, has killed more than 380,000 people in nine years and displaced around half of the country's population.

'Giant refugee camp'

The Daesh (so-called IS) group's once sprawling "caliphate" has collapsed, and the number of front lines been reduced by recent government gains.

But aid groups have warned that one of the conflict's worst humanitarian catastrophes yet could be unfolding in Idlib, with staggering numbers of families sleeping rough in sometimes sub-zero temperatures.

"You shouldn't have a war in what is essentially a giant refugee camp," the head of the Norwegian Refugee Council, Jan Egeland, told CNN in an interview.

The Idlib region is a dead-end for hundreds of thousands of people who were forced to flee or were evacuated from formerly rebel-held territory elsewhere in Syria.

Some have moved four times or more since the start of the war but there is nowhere for them to go after Idlib, with the Turkish border to the north and government forces in the other three directions.

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.

Russia-Turkey deal

With 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 Monday, artillery fire by government forces killed at least five Turkish soldiers in Idlib.

Ankara said it responded and "neutralized" 101 Syrian forces.

It was not possible to verify that claim on the ground, and neither state media nor the Syrian Observatory for Human Rights reported any casualties on the government side.

While the spat heightened fears of yet another deadly escalation in the region, analysts argued that the tense geopolitical tango between Russia and Turkey was still on.

"There's a deal between Russia and Turkey for the piecemeal retaking of Idlib in exchange for the disposal to Turkey of Kurdish areas," Fabrice Balanche, a geographer and Syria expert, said.

Turkish President Recep Tayyip "Erdogan is distrustful of Syria and Russia, who might be tempted to... seek a broader reconquest than planned," he said.

Balanche said the deal between the two powers was that the Syrian army could secure the M5 but should not yet seek to retake Idlib city.

Turkey, which already hosts the largest number of Syrian refugees, needs time to handle the huge numbers of people massing at its border, Balanche said. — AFP