BEIRUT - Warplanes carried out Sunday air raids on two villages in northern Syria 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 cease-fire brokered by Russia and the U.S., the most ambitious effort yet to curb the violence of Syria'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.

In Damascus on Saturday, after the partial cease-fire went into effect, CBS News continued to hear sporadic shelling and mortaring from the suburbs. But overall, Day 1 went better than anyone expected -- although there is no official monitoring.

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.

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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 of Iraq and Syria (ISIS) are excluded from the truce.

Opposition activists and state media also reported clashes between troops and members of ISIS mostly in the northern province of Aleppo.

Meanwhile, the Russian military operating in Syria said it has information about an attack on the Syrian border town of Tal Abyad from Turkish territories with the use of large-caliber artillery and has asked the United States for an explanation.

Russia has set up a center for monitoring the truce at the Hemeimeem air base in Syria, where Russian warplanes are based.

On Saturday, members of ISIS attacked the border town of Tal Abyad and the nearby village of Suluk that were captured months ago by Kurdish fighters, according to a Syrian rebel official.

Hours-long battles between Kurdish fighters and ISIS militants forced them out of Tal Abyad and other areas despite some presence of the extremists on the outskirts of the border town.

The Observatory said the battle of Tal Abyad lasted a full day and left 70 ISIS fighters, 20 Kurdish fighters and 10 civilians dead.

The head of the center. Lt. Gen Sergei Kuralenko, said Sunday that the suspected violation took place overnight and his center has turned to the corresponding U.S. center in Amman for an explanation, since Turkey is a member of the U.S.-led coalition, Russian news agencies reported.

In Turkey, President Recep Tayyip Erdogan said the cease-fire aiming to reduce the violence in Syria is only being partially implemented.

Erdogan expressed hope that "today or tomorrow this cease-fire will be secured and calm prevails in Syria" after noting that it is only being adhered to "in about one-third" of Syrian territories.

Erdogan made the remarks at a news conference in Istanbul prior to embarking on a trip to Africa.

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