The Turkish army is amassing tanks and commandos ahead of a joint mission with Russia and Iran to monitor a ceasefire agreement and pacify a rebel stronghold in northwest Syria.

The military build-up has gained impetus since the three allies agreed to establish a combat-free zone in the Syrian province of Idlib — largely controlled by former Al Qaida militants — and to monitor any violations by opposition groups or forces loyal to Syrian President Bashar Al Assad. Solid steps will be taken in the coming days following discussions between the parties this week, Ebrahim Kalin, spokesman for Turkish President Recep Tayyip Erdogan, told state-run TRT television on Thursday.

Turkish troops are expected to be deployed inside Idlib with Russians stationed around the city, and the collaboration symbolises the closer ties between Erdogan and Russian President Vladimir Putin. It also represents a shift in Turkey’s attitude to Al Assad, analysts said. Turkey has long opposed any political transition under him, but Russia’s intervention in Syria’s civil war shored up the president after years of US insistence that he must go.

By joining the Idlib mission, Turkey is “de facto agreeing to the transition of power for Al Assad,” Talha Kose, an analyst with the Ankara-based Foundation for Political and Social Research, said at a conference on Syria in Istanbul on Thursday. “It is a very risky area — Turkey may face a backlash from moderate rebels if it can’t deliver humanitarian services” and may also “come under pressure” from Russia and Iran to eliminate militants, he said.

Turkey’s mission is to prevent violations of the ceasefire agreement, deliver aid to civilians and pacify groups including the Hayat Tahrir Al Sham, Kalin said, referring to the Al Qaida-linked militants. It is unclear where Iranian forces would be stationed.

A successful mission would help Al Assad focus on fighting Daesh in eastern Syria, Emre Ersen, an analyst at Istanbul’s Marmara University, said at the conference in the Turkish capital. In addition to HTS fighters, Idlib contains factions of the Turkey-backed Free Syrian Army who fought against the Syrian president.

Turkey should rely on the moderate rebels in Idlib, including the Free Syrian Army, to fight the Islamic militants and avoid any unilateral military action that could be construed as violating the terms of the agreement, said Kose.

The scale of Turkey’s military build-up has also raised questions about whether Turkey has ambitions after Idlib, particularly in the neighbouring Kurdish-run province of Afrin. Turkey has threatened to clear the province of Kurdish forces that it regards as belonging to a terrorist group with links to the PKK, which has long battled for autonomy in Turkey’s southeast.

Turkey’s Syria policy “may be serving other goals beyond Idlib,” said Hakki Uygur, an analyst at the Centre for Iranian Studies in Ankara. Turkey is seeking the support of both Russia and Iran against the Kurds in Syria, he said.

Tensions between Nato allies Turkey and the US have been exacerbated by Washington’s decision to send arms to the Kurds to aid the fight against Daesh.