Government shelling killed three children and two women on Saturday in Syria's rebel-held Idlib province, where Russia and Turkey agreed on a buffer zone, a monitoring group said.
The UK-based Syrian Observatory for Human Rights said shells hit Jarjanaz village which lies on the inner edge of the 15-20 km (9-13 mile) deep zone.
The deal to create the demilitarized zone, running along the contact line between insurgent and government territory, staved off an army offensive against Idlib.
Intermittent exchanges of fire have broken out in northwest Syria since the agreement between Russia, a key Damascus ally, and Turkey, which backs some rebel factions.
The United Nations says nearly 3 million people live in the northwest region and has warned against a battle to restore state rule there.
Among an array of factions controlling Idlib, the dominant force is Tahrir al-Sham, an Islamist alliance led by fighters formerly linked to al Qaeda.
Earlier this month, Moscow accused insurgents of trying to wreck the deal, while rebels accused the Syrian army and its allies of attacking the region.