Opposition monitoring groups say warplanes have carried out air raids on two villages in northern Syria.

Sunday's air raids came on the second day of a cease-fire brokered by Russia and the U.S., the most ambitious effort yet to curb the violence of the country's five-year civil war.

The Britain-based Syrian Observatory for Human Rights said the airstrikes hit the villages of Daret Azzeh and Qobtan al-Jabal. The group did not say whether the warplanes were Russian or Syrian.

The Local Coordination Committees said the warplanes were Russian.

It was not immediately clear if the warplanes struck areas controlled by Al Qaeda's branch in Syria, known as the Nusra Front. Both the Nusra Front and the Islamic State group are excluded from the truce.