Russian warplanes early Wednesday carried out their first airstrikes in three months on the last remaining opposition stronghold in northwestern Syria, opposition activists have said.

The activists said the strikes on and near Idlib province were the first to be conducted by Russian planes since a truce brought relative calm to the volatile region in early March. There was no immediate word on any casualties.

Idlib is currently home to about 4 million civilians, including hundreds of thousands displaced in recent years by regime forces throughout war-torn Syria, with some likely using the cease-fire as an opportunity to return home.

Around 1 million Syrians were displaced from Idlib province when the Bashar Assad regime and its allies launched an offensive last November.

Most of the refugees sought shelter at camps close to the border with Turkey, while others went to areas under the control of the Syrian opposition.

The March 6 protocol between Ankara and Moscow urged all parties to halt fighting in the de-escalation zone.

Opposition activists said that in recent days, both Turkey and the government in Damascus have been sending reinforcements to northwestern Syria. There have also been repeated violations of the truce on the ground in recent weeks.

The U.K.-based Syrian Observatory for Human Rights, an opposition war monitoring group, said Russian warplanes targeted opposition-held areas just south of Idlib province late Tuesday and Wednesday morning. It had no immediate word on casualties in the four airstrikes.

Idlib-based activist Taher al-Omar said Russian warplanes fired missiles that caused wide destruction. He also gave no word on casualties.

Another Idlib-based activist, Hadi Abdullah, said Wednesday’s strikes hit a mountain region in Latakia province on the edge of Idlib, as well as a power station on the northern edge of Hama province.