The violence was the latest in a spate of violations of a ceasefire deal that was brokered by Turkey and Russia in March 2020 and had largely held since.
Monday's artillery fire struck several locations near the ceasefire lines and one Syrian soldier was killed in retaliatory fire, the Syrian Observatory for Human Rights said.
In one incident in the village of Al-Bara, two women were killed by regime shelling, the Observatory said.
In the village of Ihsem, shelling struck a police station, killing one policeman and four members of armed groups opposed to the government of President Bashar al-Assad and its allies.
Another 13 people were wounded, the monitor said.
Both areas are under the control of Hayat Tahrir al-Sham, a jihadist organisation that includes ex-members of Al Qaeda's former Syria affiliate.
Smaller factions are also present in those areas, said Rami Abdel Rahman, the head of the UK-based Observatory.
The northwestern Idlib region, which borders Turkey to the north and is home to more than a million people, is the last part of Syria controlled by rebel or jihadist groups.
Assad's regime, backed by Russia and Iran, has vowed to retake the region and the enclave shrank under pressure from deadly land and air offensives.
A ceasefire deal brokered by Ankara and Moscow was reached 15 months ago.
Despite sporadic skirmishes along the ceasefire lines, the truce has largely held, averting a major assault that aid groups warned could cause suffering on a scale yet unseen in the decade-old war.
The past few weeks have witnessed an uptick in violations, mostly by regime and allied forces, Abdel Rahman said.
"The escalation is unprecedented since the start of the truce," he said.
On June 10, 12 people were killed in regime shelling on the Idlib region, in what was one of the deadliest incidents since the ceasefire came into effect.
According to the Observatory, close to half a million people have been killed by conflict since March 2011, when brutal government repression of pro-democracy protests ignited a civil war that also displaced half of Syria's population.