Turkey sent special forces and commandos over the border into Syria, the start of a joint mission with Russia and Iran whose stated goal is to monitor a cease-fire agreement and pacify a stronghold for Islamic militants – but one that also has major implications for the region’s Kurds.

Turkish troops began establishing observation points in Idlib on Thursday, the army said on its website. The mission is being conducted in line with rules of engagement agreed with Russia and Iran, it said without elaborating.

The effort to establish a combat-free zone in Syria’s northwest province of Idlib, which is largely controlled by former al-Qaida militants, began late Thursday when 30 armored cars carrying more than 100 special forces and commandos crossed the frontier, Hurriyet newspaper reported, adding that more soldiers are expected to be deployed in coming days.

“We said that we could come suddenly one night, and as you know, last night our armed forces started an operation in Idlib,” President Recep Tayyip Erdogan said in a speech in Ankara to provincial leaders from his party. “No one can ask us why we’re doing this. We’re the ones who have a 911-kilometer border with Syria. We’re the ones who are constantly under threat.”

The incursion is one of Turkey’s biggest moves yet in the Syria conflict, adding to the nation’s geopolitical risks following an attempted coup in 2016 and subsequent political crackdown. The lira tumbled after the U.S. and Turkey suspended visa services for each other’s citizens Sunday, a sharp escalation of simmering tensions between the two over issues including U.S. military involvement in Syria, Turkey’s closer ties with Russia, the trials of Turkish businessmen in the U.S. for alleged violations of Iranian sanctions, and Turkey’s jailing of U.S. citizens and Turkish employees of the U.S. diplomatic mission.

Turkish troops entered Syria from areas evacuated by the group called Hayat Tahrir al-Sham following negotiations with the al-Qaida linked alliance, Hurriyet said.

Turkey’s deployment emphasizes the closer ties between Erdogan and his Russian counterpart Vladimir Putin, a relationship viewed with concern by Turkey’s NATO allies.