Syrian activists said a wave of U.S.-led airstrikes has been carried out against Islamic State militants in and around the city of Raqqa, the self-proclaimed capital of the jihadist group.
The Britain-based Syrian Observatory for Human Rights said Sunday as many as 30 strikes hit overnight and targeted Islamic State positions in Raqqa and the Division 17 air base, which the militants seized earlier this year from government forces.
Observatory director Rami Abdel Rahman said it has been "a long time" since this large a number of targets have been hit.
A casualty toll was not immediately available.
Turkey said Saturday a Kurdish claim that Islamic State militants attacked the Syrian border city of Kobani from the Turkish side of the border is a lie.
A statement from Prime Minister Ahmet Davutoglu's office said Turkish security forces have taken "all necessary precautions" along the border.
A Kurdish official and the Syrian Observatory for Human Rights said a vehicle used in a car bombing in Kobani Saturday crossed over the border from Turkey.
Kurds also claim the militants are using grain silos in Turkey from which to launch attacks on Kobani.
Ankara has consistently denied Kurdish assertions that it is cooperating with the Islamic State group, saying the group is also a threat to Turkey's security.
Turkey has so far been reluctant to join the U.S.-led coalition fighting to push back Islamic State fighters inside Syria.
Kurdish forces, backed by coalition airstrikes, have been battling the Islamic State group for control of Kobani for more than two months with neither side gaining a big advantage.