Turkey has sent significant numbers of tanks and troops deep into Syria in an apparent bid to reinforce the town of Khan Sheikhun, where 92 people died in a 2017 sarin gas attack, one of the war’s most infamous atrocities.

Columns of Turkish forces, amounting to Ankara’s largest incursion into Syria of the eight-year conflict, crossed the southern border into Idlib province early on Monday, accompanied by Arab proxy forces.

Their advance was slowed by regime airstrikes that stopped them reaching the town in southern Idlib, where a battle continues to rage between anti-Assad groups and forces fighting on behalf of the Syrian leader.

Ankara decried the airstrikes, and claimed its convoy was travelling towards outposts already established in the region under an agreement with Russia. Turkey claimed the airstrike hit its convoy, killing three civilians, however a war monitor said a Russian airstrike took the lives of three rebels in the surrounding area.

The intervention comes at a pivotal time in the fight for Idlib – the last corner of Syria without a regime presence – and was seen as a move to bolster a withering armed opposition and a beleaguered population of more than 3 million people. Syrian officials claimed the Turkish convoy was transporting ammunition and supplies to armed groups.

Until recent weeks, a Russian-led offensive on Idlib launched in late April had made little progress, with opposition groups, among them jihadist factions, holding ground in the south of the province despite relentless barrage from the skies.

However, momentum has shifted in the past week, with regime ground forces ascendant and Russian jets stepping up bombing runs that have obliterated towns and villages beneath them.

The United Nations estimates that up to 400,000 people have been newly displaced inside Idlib. Many of those once again on the run are sleeping in the open with no shelter and little access to medical care.

Hospitals and health clinics have been systematically targeted by airstrikes, severely limiting access to life-sustaining aid. At least 881 civilians and more than 2,000 combatants have been killed on the insurgent side in the past four months, while at least 1,400 pro-Assad troops are also thought to have been killed.

“Using airstrikes and artillery, the regime is advancing by flattening everything in its path,” said James Le Mesurier, the founder of Mayday Rescue, which runs the White Helmets civil defence organisation.

“The towns and villages that it is proclaiming to have liberated are empty of all civilians who fled ahead of the fighting, with what possessions they can carry. Close to half a million people have been displaced in this current round of fighting. There are increasingly fewer places that they can escape to.”

Idlib has become the last redoubt of those who rose up against the Syrian dictator during the Arab revolts of 2011. As Syria’s towns and cities have been pummelled and the regime – heavily backed by Iran and Russia – has clawed back early losses, displaced populations from all corners of the country have ended up in the north-west province.

Among them are large numbers of extremists who established a foothold in Idlib in mid-2012 and with whom rebel groups and proxies backed by neighbouring states earlier in the conflict have been forced to co-exist.

Winning back Idlib has become a primary goal of the Syrian leader and Russia – and to a lesser extent, Iran, which has extensively committed troops and proxies elsewhere in the war. Both sides have saved the central government from battlefield defeat.

The Russian president, Vladimir Putin, on Monday reaffirmed his support for the Idlib campaign during a meeting with Emmanuel Macron in Paris. The French leader, meanwhile, urged for a recent ceasefire to once again be respected.

Turkey’s intervention adds a new dimension to an already intractable conflict, and one of the world’s gravest humanitarian crises, which shows little sign of ending.

While no longer robustly supporting the anti-Assad opposition, and now framing its involvement in Syria through its own interests, Ankara has insisted that it would not let the province fall militarily.

To do so, would inevitably send tens of thousands of refugees towards its borders, at a time when Turkish authorities have been rounding up and deporting Syrian citizens in Istanbul and cities closer to the border.

“We have moved to a narrower role, said one senior Turkish official. “We have our own interests in the outcome, which we will continue to shape.”

Khan Sheikhun has become a focal point of the fight for Idlib because of its position on a main highway linking Idlib city with Hama to the south. In April 2017, a sarin attack struck the centre of the town, killing 92 and maiming more than 200 people. The Organisation for the Prevention of Chemical Weapons (OPCW) and a UN investigation blamed a Syrian regime jet for the strike. Donald Trump ordered the bombing of the base that the jet took off from in retaliation.

“At least they only dropped one bomb on us then,” said Mabrouk Falah, who fled Khan Sheikhun with his family as fighting escalated last week. “Now it is just raining bombs, never ending. Everything is targeted, even the ants. And they wonder why we resist? And they wonder why we don’t discriminate about who fights for us. The world has looked away, and we must survive whatever it takes.”