Warplanes carried out air raids Sunday on several parts of northern Syria as a top opposition official warned that continued violations of a fragile ceasefire could jeopardize a planned resumption of UN-brokered peace talks.

The acts of violence came as Russia said a northern town held by a predominantly Kurdish militia came under fire from the Turkish side of the border.

Sunday's air raids came on the second day of a ceasefire brokered by Russia and the United States, the most ambitious effort yet to curb the violence of the country's five-year civil war. The truce has been holding since it went into effect at midnight Friday despite accusations by both sides of violations.

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

The local co-ordination committees said the warplanes were Russian.

The Observatory and the LCC also reported air raids on the northwestern town of Jisr al- Shughour saying a woman was killed and 12 others wounded.

It was not immediately clear whether the warplanes struck areas controlled by al-Qaeda's branch in Syria, known as the Nusra Front. Both the Nusra Front and the Islamist radicals ISIS are excluded from the truce.

In another development, Syria's state news agency said militants fired shells into government-held areas in the coastal province of Latakia from their bases near the Turkish border. The agency reported that the shelling killed and wounded a number of people, without giving details.

Opposition activists and state media also reported clashes between troops and members of ISIS, mostly in the northern province of Aleppo. Nevertheless, both sides have said they will continue to abide by the truce.

Also Sunday, Riad Hijab, who heads the High Negotiations Committee, an umbrella for opposition and rebel factions, said in a statement directed to UN chief Ban Ki-Moon that Russian, Iranian and government forces have not stopped hostilities since the truce went into effect.

Hijab said there have been 24 cases of shelling and five cases of ground attacks. He added that Russian warplanes carried out 26 airstrikes on Sunday alone targeting rebels abiding by the truce.

The UN Special Envoy to Syria Staffan de Mistura has called for a new round of indirect peace talks in Geneva on March 7, after the first round of talks collapsed earlier this month.

The Syrian conflict has killed an estimated 250,000 people, displaced half the country's population and triggered one of the worst refugee crises since the Second World War.