SALHIYAH, Iraq — Kurdish fighters in northern Iraq captured a border crossing with Syria on Tuesday, expelling Islamic State militants in heavy fighting that ground down to vicious house-to-house combat and close quarters sniping.

In neighboring Syria, Kurdish militiamen were on the defensive as the extremists pressed ahead with a relentless assault on a town near the Turkish border. The attack on Kobani, also known as Ayn Arab, has driven more than 160,000 people across the frontier in the past few days.

Iraqi Kurdish fighters, known as peshmerga, were doing the bulk of the fighting on the ground as a U.S.-led coalition carried out an aerial assault against the Islamic State group in both Iraq and Syria. Britain joined the air campaign Tuesday, carrying out its first strikes against the extremists in Iraq – though it does not plan to expand into Syria.

TO NEGATE A CALIPHATE

The goal of the campaign is to push back the militant group that has declared a self-styled caliphate, or Islamic state, ruled by its brutal interpretation of Islam in territory it has seized across much of Iraq and Syria.

The Kurdish fighters in Iraq said they saw some of the heaviest fighting yet. Peshmerga spokesman Halgurd Hekmat told The Associated Press the Kurds seized the border crossing of Rabia, which the extremists captured in their blitz across Iraq over the summer.

Rami Abdurrahman, the director of the Britain-based Syrian Observatory for Human Rights, also said the Kurds had retaken the border post.

Kurds wounded in the fighting were brought to a makeshift clinic in the town of Salhiyah, where dusty and exhausted, they described savage battles, with militants sniping at them from inside homes and from the windows of a hospital in Rabia.

U.S. WEAPONS FALL INTO WRONG HANDS

“They’re such good fighters,” said one soldier, resting outside the clinic on a rock surrounded by blood-soaked bandages. “They’re fighting with weapons the Iraqi military abandoned – so, American weapons really.”

In northern Syria, Islamic State group fighters have pushed ahead with an assault for days trying to take the beleaguered Kurdish town of Kobani near the Syrian-Turkish border – despite U.S. airstrikes on their positions.

The fighting has created one of the single largest exoduses in Syria’s civil war, now in its fourth year: More than 160,000 people have fled into Turkey over the past few days, the U.N. humanitarian chief Valerie Amos said.

“Their fear is so great that many people crossed heavily mined fields to seek refuge,” she told the U.N. Security Council.

In Britain’s first airstrikes of the campaign, two Tornado jets hit a heavy weapons post and an armored vehicle being used by the militants to attack Kurdish forces in northwest Iraq, British Defense Secretary Michael Fallon said.