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 because the UN has deemed them to be "terrorist organizations."

Syria's opposition will stick to the cessation of hostilities despite at least 15 violations, a spokesman for the High Negotiating Committee (HNC) said on
Sunday.

Salim al-Muslat said the HNC, the main Syrian opposition group that attended peace talks in Geneva earlier in the month, would complain to the United
Nations and countries backing the peace process about alleged Russian air strikes around the city of Aleppo and attacks by Hezbollah in the town of Zabadani, without giving details.

The opposition is waiting for answers about how the cessation of hostilities in Syria, which came into effect at midnight on Friday, is being monitored, he said.       

With files from Reuters