Syrian opposition commanders have said a counteroffensive against government forces in the country's northwest could start "at any moment" amid escalating tensions in the region.

The warning on Monday came hours after Ankara said five Turkish troops were killed in shelling by the Syrian army in Idlib province, the opposition's last stronghold in the country. The Turkish army retaliated by hitting more than 100 Syrian military targets, according to the Turkish defence ministry.

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These developments come as reinforcements of fighters from the Turkey-backed Syrian National Army (SNA) and the Syrian Liberation Front (SLF) have been sent over the past few days to the front line in western Aleppo and southern Idlib provinces in response to the advances of the Russia-backed Syrian army.

Thousands of Turkish soldiers have also been deployed to observation posts and along the front line, with several convoys with armoured vehicles, battle tanks and artillery entering northwest Syria over the past week.

On Monday morning, Turkish artillery started shelling positions held by Syrian government forces in the city of Saraqeb, western Idlib province, but the attack stopped shortly after.

"The operation [against the government forces] was supposed to start at 10am today. However, it was stopped due to the arrival of a Russian delegation to Ankara," said one SNA commander, who asked not to be named, as he was not authorised to discuss the matter.

A delegation from Russia headed by Sergey Vershinin, deputy minister of foreign affairs in charge of Syrian affairs, and Alexander Lavrentiev, the special envoy of the Russian presidency to Syria, is in Ankara for a series of meetings with Turkish officials.

According to the commander, the negotiations failed and currently, the Turkish government is waiting for a visit from James Jeffrey, the US special representative for Syria, before it pushes forward with the operation. Its aim will be to reclaim territories the Syrian government has taken over since December last year.

On Wednesday, Turkish President Recep Tayyip Erdogan gave an ultimatum to the Syrian government to withdraw its forces from Turkey's observation posts in Idlib province. His warning came two days after Syrian government forces targeted a Turkish convoy carrying reinforcements to observation points in territory currently controlled by Damascus. The attack killed seven Turkish soldiers and a contractor, angering Ankara.

The situation escalated on Monday with the attack by Syrian government forces on a new observation point the Turkish army had set up in Taftanaz military base, 15km (9.3 miles) northeast of Idlib city. A further five Turkish troops were wounded in the deadly attack.

Khalid Rahal, another commander of the opposition forces, also confirmed that there have been serious preparations for a military operation led by the Syrian opposition and backed by the Turkish army during the past week.

"Given the current situation and in view of the killing of Turkish soldiers and attack on the Turkish convoy [last week], a battle can break out at any moment," he told Al Jazeera.

He said the operation will focus on retaking some areas from the Syrian government in Idlib and Aleppo provinces.

According to Can Kasapoglu, the director of Security and Defense Studies Program at Istanbul-based think-tank EDAM, the deployment of Ankara's forces on the ground and the set-up of new Turkish defence positions, apart from its 12 observation points, point to an imminent military escalation.

"Turkey is deliberately escalating the situation militarily to force Russia to broker a ceasefire between the Syrian Arab Army and Turkey because this is the only way in the eyes of Ankara to stop the regime offensive," he told Al Jazeera.

In his view, a military escalation is quite likely but it will be limited in scope and it would aim to stop the advance of the Syrian government.

"There can be a counteroffensive, but it would be part of a larger defensive [strategy] for Idlib," he said, adding that a wider operation to retake territory from the Syrian government forces is unlikely given the fact that the Turkish army cannot provide air cover for the opposition forces.

According to Kasapoglu, the aim of the Syrian government is to "depopulate" opposition-controlled northwest Syria in order to be able to control it. Turkey fears such a development as the more than three million people residing in the area will head to its border with Syria or to the area under its direct military control in northern Aleppo province, Kasapoglu said.

On Monday, the United Nations said that some 700,000 people have been displaced since December in northwest Syria, some 100,000 of them in the past week alone. Humanitarian workers have warned that the situation in the region is worsening and that there are not enough tents and food for the growing number of displaced people.