Islamic State fighters have killed at least 146 civilians during an offensive on the Kurdish-held town of Kobani, according to a group monitoring the conflict.
The fighting began early Thursday when Islamic State fighters disguised as Kurdish soldiers detonated a car bomb near the town's border crossing with Turkey.
Rami Abdel Rahman, who heads the Britain-based Syrian Observatory for Human Rights, told VOA the fighting was continuing as of early Friday.
"One hundred and forty-six (civilians have been killed) between Kobani and around Kobani. There are still some ISIS inside the city," Abdel Rahman said, using an acronym for the Islamic State group.
Rahman said the Islamic State fighters are holed up at three points inside Kobani, where they are thought to be holding hostages. Dozens of Islamic State fighters also have been killed.
It is the Islamic State's second-largest massacre of Syrian civilians since the conflict began over three years ago, according to the Observatory.
The largest massacre occurred in August, when the extremist group executed 700 members of the al-Sheitaat tribe in eastern Syria.
The flashpoint town of Kobani holds important symbolic value. Kurdish fighters backed by U.S.-led airstrikes took control of the town in January following months of fierce clashes with Islamic State forces.
A Syrian Kurdish politician, Gelo Isa, told VOA that Islamic State fighters carried out multiple suicide attacks in and around Kobani on Thursday, killing 23 civilians in one attack.
Isa also said a group of IS fighters entered Kobani from Turkey and "randomly killed civilians who came out to see what was going on." He said the militants wore Kurdish military uniforms and carried Free Syrian Army flags in an effort to deceive security guards.
A journalist based in Kobani, Zara Musto, told VOA that IS vehicles with fighters wearing Kurdish uniforms arrived at a checkpoint south of the city Thursday. Musto said guards let the vehicles pass.
Turkey has denied claims that any Islamic State fighters crossed from Turkey Thursday in an attack on Kobani.
The attack came after Islamic State militants suffered a series of setbacks in northern Syria against Kurdish forces in recent weeks.
Fighting in Syria has killed more than 220,000 people since the conflict began in March 2011.