BEIRUT (AP) Islamic State fighters backed by tanks have captured 21 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 extremists sweeping through the area, activists said.

For more than a year, the Islamic State group 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 U.N. says has killed more than 190,000.

Since Wednesday, Islamic State militants appear to have gained the upper hand in Syria's northern Kurdish region of Kobani, also known as Ayn Arab, overrunning 21 Kurdish villages, the Britain-based Syrian Observatory for Human Rights said. It said there were casualties on both sides, but that Kurdish civilians were fleeing their villages for fear that Islamic State group fighters "will commit massacres against civilians."

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

"The battles that are taking place in Kobani are the most violent," Khalil said adding that Islamic State group fighters were using tanks in their offensive. Khalil called on Kurds around the world to come to Syria to defend Kobani.

The fighting forced nearly 3,000 people to try flee to Turkey and gathered near the border Turkish district of Suruc, according to the private Dogan News Agency. A video released by the agency showed Syrian refugees walking to the border with some Kurds asking to be allowed to cross to stay with relatives on the Turkish side of the frontier.

Like many fronts of Syria's civil war, momentum in the fight between the extremists and the Kurds has swung back and forth. Earlier this week, for example, Kurdish fighters captured 14 villages from the Islamic State in other parts of Syria.

Still, the retreat in Kobani marked a setback for the battle-hardened Kurdish force known as the People's Protection Units. The militia, which also goes by the initials YPK, has been perhaps the most successful fighting force battling the Islamic State group, which has routed Iraqi and Syrian government forces. Last month, the YPK crossed the border into Iraq and opened a safe passage for members of the ancient Yazidi minority who were attacked by Islamic State fighters.

The fighting around Kobani is part of the Islamic State's wider battle in Syria as the extremists look to seize control of the few areas in the northeast still outside of their hands.

The Syrian government, meanwhile, has begun targeting the group with greater frequency since the militants overran much of northern and western Iraq. Before that, President Bashar Assad's has 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 the Islamic State group, killing more than two dozen people. The Local Coordination Committees activist group said 51 people were killed in the attack in which a helicopter dropped a barrel packed with explosives on a bakery.

The Observatory also reported the airstrike, but said at least 26 were killed. It warned that the number could rise because some of the wounded are in critical conditions.

The discrepancy in death tolls could not be immediately reconciled, but casualty figures frequently differ in the chaotic aftermath of attacks in Syria.

The U.S. has been conducting airstrikes against Islamic State fighters in Iraq since early August. President Barack Obama last week authorized strikes against the group 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 Rami Abdurrahman said activists saw drones flying over areas held by the Islamic State group, including the towns of Manbij and Maskaneh. He added that it is not clear whether the drones were American.