Opposition activists say government forces have intensified their bombardment of rebel-held towns and villages in northwestern Syria, striking a market where four people were killed.

The new wave of violence that began on April 30 is the worst since September, when Russia and Turkey brokered a cease-fire that averted a government offensive on Idlib province, the last major rebel stronghold.

Opposition-linked first responders, known as the White Helmets, say warplanes bombed a market in the village of Ras el-Ain on Tuesday, killing four people and wounding 20. The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, reported airstrikes and shelling on Idlib and northern rebel-held parts of Hama province.

The latest wave of violence was triggered by attacks by al-Qaida-linked militants that killed dozens of troops.