Rebel commanders and rights groups have said Russia and its Syrian ally stepped up aerial strikes last week in a new and bloodier phase of a three-month-old assault on the last opposition bastion in Syria’s northwest. The offensive backed by Russia has killed at least 400 civilians in northwest Syria and uprooted more than 440,000 people since April, the United Nations says.

At least four civilians have been wounded in a rocket attack on Russia’s military base in Syria, the Russian Defense Ministry has said.

The rebels launched three rockets from the buffer zone in Idlib toward Russia’s Khmeimim airbase in the nearby province of Latakia, the Defense Ministry said in a daily bulletin Monday.

No servicemen were hurt in the attack but four civilians were injured when the rockets fell on a nearby settlement, it added.

The Syrian army said on Monday it was resuming military operations in a Russian-led campaign in Idlib, blaming Turkey for not abiding by its commitments under a truce deal.

Russian jets joined the Syrian army on April 26 in the biggest offensive against parts of rebel-held Idlib province and adjoining northern Hama provinces in the biggest escalation in the war between Syrian President Bashar al-Assad and his enemies since last summer.

At least 544 civilians have been killed and over 2,000 people injured since the assault on Idlib began, rights groups and rescuers said in July.

Reuters contributed reporting to this article.