Syrian armed opposition and a war monitoring group say insurgents have launched an offensive against government forces on the edge of their stronghold in northwestern Syria.

The Syrian Observatory for Human Rights said the rebel offensive Thursday afternoon began with a volley of rockets and missiles lobbed at government forces in villages recently captured from the opposition. The Observatory said the rebels burned tires to obstruct warplanes' vision.

A rebel umbrella group, National Front for Liberation, said the offensive is aimed to repel government forces.

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.