At least three Russian and several Turkish soldiers have been wounded in Syria's Idlib province when a joint military patrol was hit by an improvised explosive device, according to Russia's defence ministry.
The device blew up at 8:50am local time (05:50 GMT) as the soldiers' convoy patrolled the M4 highway in the southern part of a de-escalation zone, the ministry said in a statement on Tuesday.
A Russian and a Turkish armoured vehicle were damaged, with three Russians "lightly injured", it added. Several Turkish soldiers were also wounded.
Russia and Turkey launched the patrols along the strategic highway in March following a ceasefire agreement aimed at stopping heavy fighting in and around Idlib, the last major bastion of anti-government forces in Syria's war.
Rami Abdel Rahman, the director of the UK-based Syrian Observatory for Human Rights (SOHR), reported that "numerous explosions were heard as the patrol passed in a village northeast of the town of Ariha".
The blast occurred "despite Turkish forces having thoroughly combed the area 24 hours before the joint patrol set off on the road separating areas controlled by the rebels from those held by government forces, according to the ceasefire", he said.
No such patrols had yet been able to circulate along the entire length of the road as stipulated in the ceasefire agreement, from the village of Tarbana in the east of Idlib province to the village of Ain Hawr in the north of Latakia province, according to the SOHR.
Fighters opposed to the patrols, as well as protests against them, had previously prevented them from advancing, but without them ever being targeted like this, it said.
Russia and Turkey have long supported opposing sides in Syria's war - Moscow is a major backer of Syrian President Bashar al-Assad while Turkey supports certain rebel groups - but the two countries have closely coordinated in the past and agreed upon several deals to reduce hostilities.
Home to some three million people, the Idlib region in northwest Syria is dominated by the Hay'et Tahrir al-Sham group, an armed group that mainly consists of members of Syria's former al-Qaeda affiliate, and its rebel allies.
A Russian-backed government offensive in northwest Syria displaced nearly a million people between December and March, but thousands have returned since the truce came into force.
After Tuesday's attack, Russian fighter jets launched several air raids on fighter positions in the countryside of Latakia province, according to SOHR.