MURSITPINAR, Turkey — Islamic State fighters backed by tanks and artillery pushed into an embattled Syrian town on the border with Turkey on Monday, touching off heavy street battles with the town’s Kurdish defenders.

Hours after raising two of their Islamic State black flags on the outskirts of Kobani, the militants punctured the Kurdish front lines and advanced into the town itself, the Local Coordination Committees activist collective and the Britain-based Syrian Observatory for Human Rights said.

“They’re fighting inside the city. Hundreds of civilians have left,” said Observatory director Rami Abdurrahman. “The Islamic State controls three neighborhoods on the eastern side of Kobani. They are trying to enter the town from the southwest as well.”

The center of the town was still in Kurdish hands, Abdurrahman said.

Since beginning its offensive in mid-September, the Islamic State group has barrelled through one Kurdish village after another as it closed in on its main target – the town of Kobani, also known as Ayn Arab. The assault has forced some 160,000 Syrians to flee and put a strain on Kurdish forces, who have struggled to hold off the extremists even with the aid of limited U.S.-led airstrikes.

Capturing Kobani would give the Islamic State, which already rules a huge stretch of territory spanning the Syria-Iraq border, a direct link between its positions in the Syrian province of Aleppo and its stronghold of Raqqa, to the east. It would also crush a lingering pocket of resistance and give the group full control of a large stretch of the Turkish-Syrian border.

As fighting raged within sight of the Turkish border, the country’s defense minister said the NATO alliance had drawn up a strategy to defend Turkey, a NATO member, if it is attacked along its frontier with Syria.