War planes attacked six towns in Syria's northern Aleppo province early on Sunday, the Syrian Observatory for Human Rights said, a day after a cessation of hostilities agreement took effect.
Syrian insurgents said the air strikes were carried out by Russian war planes in support of Syria's government, but the Observatory which monitors the conflict said the identity of the jets was not clear.
"We do not know which planes carried out the strikes and also we are not sure if this is considered a breach to the truce because it is not clear if these towns are included in the truce," the Observatory's director Rami Abdulrahman said.
Syria's state media did not mention the strikes. Russia's defence ministry declined to comment.
The cessation of hostilities, agreed as part of a US and Russian plan, does not cover assaults on militants from Islamic State or the Nusra Front—an al Qaeda affiliate that has called for an escalation of attacks on Friday.
Abdulrahman said some of the towns which were attacked, including Daret Azza, were controlled by Nusra Front and other Islamist groups.
Other attacks hit the villages and towns of Qabtan al-Jabal, Andan, Hreitan, Kfar Hamra and Ma'aret al-Arteek, the Observatory said, all in the west of the province where insurgents from the Free Syrian Army, who are covered by the truce, have operated.
Two videos sent by a rebel commander to Reuters shows a strike in another town, Harbnafseh, at 6.30am and another at 07:00am according to the voice in the video. The footage shows plumes of smoke rising into the sky.
Russia said on Saturday that it would suspend all flights over Syria for one day to ensure no targets covered by the truce were hit by mistake.