At least eight people were killed by Syrian government shelling of the rebel-held Idlib province on Friday, according to a monitoring group -- the highest daily toll since a Russian-Turkish demilitarization zone was set up around the region.
The Syrian Observatory for Human Rights said the deaths occurred in the town of Jarjanaz, which lies on the inner edge of the 15-20 km (9-13 mile) deep zone agreed in September.
The agreement, struck between Russia, Syrian President Bashar al-Assad's most powerful ally, and Turkey, which backs the rebels, staved off a government offensive to retake Idlib and adjacent opposition-held regions.
The UN says around 3 million people live in those areas and has said a battle to restore Assad's control there could be the worst of the seven-year-old war.
Russia has said rebels are trying to wreck the Russian-Turkish initiative and Damascus has said Turkey seems unwilling to implement it. But Turkey says the deal is going to plan.
The Observatory, a British-based monitoring group, has reported regular exchanges of fire between government and rebels since the September deal. At least 18 people have been killed by government shelling since then.
Idlib is held by an array of rebels, the most powerful being Tahrir al-Sham, an amalgamation of Islamist groups dominated by the former Nusra Front, an al Qaeda affiliate until 2016.
On Thursday, Tahrir al-Sham said it had raided government positions in a village in eastern Idlib province in response to government attacks. In a statement on its Telegram channel, it said it had killed 10 people, injured others and captured ammunition.