BEIRUT (AP) — Government airstrikes and artillery attacks ceased Saturday following a unilateral cease-fire in the northwestern province of Idlib, a Syrian war monitor said.

The Britain-based Syrian Observatory for Human Rights said it registered no airstrikes since government forces put into effect the cease-fire after midnight Saturday. The military said it agreed to a cease-fire while reserving the right to respond to any violation.

Later on Saturday, the Observatory reported a missile attack on a meeting of militants in Idlib province that killed at least 40 fighters. It said it was not clear who was behind the strike, which targeted a meeting held by leaders of militant groups including Hurras al-Deen and Ansar al-Tawhid, or whether the missiles were launched from warplanes or positions on the ground.

The Observatory suggested a U.S.-led coalition airstrike may have been behind the attack. There was no immediate confirmation of the attack or who was behind it.

Idlib province is dominated by al-Qaida-linked militant factions. It is the last bastion held by the opposition in Syria after President Bashar Assad's forces, backed by Russia, recaptured most other provinces and key cities.

The Idlib cease-fire brings temporary respite after a crushing offensive by Syrian troops in the last remaining rebel stronghold in the country. The offensive began April 30 and intensified in recent weeks, forcing hundreds of thousands to flee, many of whom were already displaced.

The U.N. said more than 450 civilians have been killed.

A similar cease-fire at the beginning of the month lasted a few days, after which the government assault resumed and forces captured the Idlib town of Khan Sheikhoun and all rebel-held towns and villages in nearby Hama province.

Also on Saturday, Syria's state-run media said troops uncovered a mass grave believed to contain the remains of Syrian soldiers killed by "terrorist groups" in the village of Latamneh, in Hama province.

SANA said 10 were retrieved from the grave, adding that they were taken to a nearby hospital to be identified.

Around half a million people have been killed in Syria's long-running civil war.

According to the U.N., more than 100,000 people have been detained, abducted or gone missing in Syria. The government is believed responsible for the large majority of those disappearances, starting with arrests during its brutal crackdown on anti-government protests that erupted in 2011.