A female Kurdish fighter carried out a suicide bomb attack against militants of the Islamic State group near the besieged Syrian border town of Kobane on Sunday, killing an unknown number of people, according to the Syrian Observatory for Human Rights.

The United Kingdom-based monitoring group said that the attack had been carried out in the eastern outskirts of the Kurdish-populated town of Kobane, following fierce clashes between Kurdish YPG fighters and ISIS militants.

“The operation caused deaths, but there is no confirmed number,” Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, reportedly said, adding that this was the first instance of a female Kurdish fighter carrying out a suicide attack against the Islamic State group.

The town of Kobane, located in the north along the border with Turkey, has been under constant attack from militants of the Islamic State group since Sept. 16. The attacks reportedly intensified on Sunday, even as aircraft belonging to the United States-led coalition pounded ISIS positions near the town.

“They are not enough to defeat terrorist groups. The strikes are helping but we need heavy weaponry, armored vehicles, cannons, rockets. They are one kilometer away in some places and two or three kilometers in others,” Idris Nahsen, a Kobane local official, told Agence France-Presse, adding that a lack of coordination between coalition forces in the air and Kurdish fighters on the ground was reducing the effectiveness of the attacks against the group formerly known as ISIS.

The protracted fighting over the control of the strategic town has prompted an exodus of more than 180,000 civilians, most of them Kurds, across the border to Turkey, according to media reports.

Meanwhile, parents of American aid worker Peter Kassig, who was threatened in a video released by the Islamic State group on Friday showing the beheading of Alan Henning, released photos and parts of a letter written by him and appealed for his release.

“We have decided to release portions of his June 2014 letter, so the world can understand why we and so many people care for him and admire him…We continue to pressure the government to stop its actions and continue to call on his captors to have mercy and release him,” Ed and Paula Kassig said, in a statement released Sunday.