BEIRUT — Syrian government forces and rebels clashed across the north on Sunday in the latest indication that a monthlong cease-fire may be breaking down, as the Islamic State seized two villages along Syria’s border with Turkey from other insurgents.

Syrian rebels advanced on government positions in the Turkmen Mountains of the northwest Latakia province, a stronghold of President Bashar Assad, according to the Britain-based Syrian Observatory for Human Rights, which relies on activists inside the country.

A coalition of Islamist and non-Islamist factions led by the jihadist Ahrar al-Sham, alongside Al Qaeda’s Syrian affiliate, the Nusra Front, seized the village of Baydaa as heavy fighting continued throughout the day, the Observatory said.

The Local Coordination Committees, another activist group, confirmed the advance and said nine government soldiers were killed.

The advance builds on a week of heavy fighting that has raised concerns about the durability of a US and Russia brokered ‘‘cessation of hostilities’’ that took effect in late February, bringing relative calm to much of the country. The Nusra Front and the Islamic State group are excluded from the cease-fire.

Rami Abdurrahman, head of the Observatory, said fighting is intensifying on all fronts around the northern city of Aleppo, adding that it ‘‘definitely’’ threatens the cease-fire.