Fighters associated with the Islamic State of Iraq and the Levant (ISIL) have captured 16 Kurdish villages over the past 24 hours in northern Syria near the Turkish border, prompting civilians to flee their homes amid fears of retribution by the group, sources said Thursday.

Rami Abdulrahman, founder of the Britain-based Syrian Observatory for Human Rights, said the villages had been seized in an ISIL advance that started on Wednesday. "They have a large number of fighters," he told Reuters by phone.

Ocalan Iso, a commander in the Kurdish People's Protection Unit (YPG), told Reuters that ISIL fighters were using heavy weapons, including tanks, in the offensive.

For more than a year, ISIL and Kurdish militias have been locked in a fierce fight in several pockets of northern Syria where large Kurdish populations reside. The clashes are but one aspect of Syria's broader civil war — a multilayered conflict that the United Nations says has killed more than 190,000 people.

Nawaf Khalil, a spokesman for Syria's powerful Kurdish Democratic Union Party, said the Kurdish fighters withdrew or lost up to 20 villages in the Kobane region and evacuated civilians within them.

"The battles that are taking place in Kobane are the most violent," said Khalil. He called on Kurds around the world to come to Syria to defend Kobane.

Like many fronts of Syria's civil war, momentum in the fight between ISIL and the Kurds has swung back and forth. Earlier this week, for example, Kurdish fighters captured 14 villages from ISIL in other parts of Syria. Now, the Kurds have been forced out of villages elsewhere.

Still, the retreat marked a setback for the battle-hardened YPG. The militia has been perhaps the most successful fighting force battling ISIL. Last month, the YPG crossed the border into Iraq and opened a safe passage for members of the Iraqi Yazidi minority who were attacked by ISIL fighters.

The Syrian government, meanwhile, has started targeting ISIL with greater frequency since the militants overran much of northern and western Iraq. Before that, President Bashar al-Assad had largely left the group alone, instead focusing his firepower on more moderate rebel brigades.

On Thursday, government helicopter gunships attacked the northern town of al-Bab, which is controlled by ISIL, killing at least a dozen people, the Local Coordination Committees activist group said. It said a helicopter dropped a barrel packed with explosives on a bakery.

The Observatory reported that at least 17 people were killed in the airstrike, but warned that the number could rise because some of the wounded are in critical conditions.

The United States has been conducting airstrikes against ISIL fighters in Iraq since early August. The U.S. House of Representatives backed President Obama's plan on Wednesday to train and arm moderate Syrian rebels, though the measure does not include any money to pay for the arms and training.

Last week, Obama authorized strikes against ISIL in Syria as well, and his administration is currently trying to cobble together an international coalition to go after the group. The U.S. is already flying reconnaissance missions over Syria.

Observatory director Abdurrahman said activists saw drones flying over areas held by ISIL, including the towns of Manbij and Maskaneh. He added that it is not clear whether the drones were American.

Al Jazeera and wire services