The war monitoring group said rebel fighters advanced in one of the villages — Al-Jebeen — after intense clashes. The government and its ally Russia responded with a wave of airstrikes, including dropping barrel bombs on villages nearby, the Observatory said.
A rebel umbrella group, National Front for Liberation, confirmed the offensive, saying it aimed to repel government forces.
Syrian state media said the insurgents shelled residential homes in the northern countryside of Hama, an area controlled by government forces, causing damages. Syrian State news agency SANA said army units responded to the source of the rockets with artillery.
Human Rights Watch said the Syrian-Russian military alliance has used internationally banned indiscriminate weapons, such as barrel bombs and incendiary weapons, in populated areas in northwest Syria in recent weeks. More than 3 million people live in the rebel-held area, nearly half of them already displaced from other rounds of violence.
Government forces began a ground offensive in late April against areas under rebel control since 2015, after a Russia-Turkey-backed cease-fire in place for months failed to reduce the influence of extremists in the stronghold or open a highway that links government-controlled cities.
Some areas in the stronghold have been in rebel hands since as early as 2011, when the civil war began.
Idlib province is the last area standing in President Bashar Assad's way as he seeks a final victory against the armed opposition after eight years of civil war. Besides Idlib, other parts of Syria in the northwest and northeast remain out of his control, held by Turkey-backed fighters or Kurdish-led forces backed by the United States.