Mürşitpınar and Beirut, Reuters—Islamic State of Iraq and Syria (ISIS) fighters advanced into the south west of the Syrian Kurdish town of Ain Al-Arab, overnight, a monitoring group said on Tuesday, taking several buildings to gain attacking positions from two sides of the city.

The prospect that the town on the Turkish border—also known as Kobani to the Kurds—could fall to militants who have besieged it for three weeks has increased pressure on Turkey, which has the strongest army in the region, to join an international coalition to fight against ISIS.

From across the nearby Turkish border two ISIS flags could be seen flying over the eastern side of Ain Al-Arab. Two air strikes hit the area and sporadic gunfire could be heard.

ISIS fighters were using heavy weapons and shells to hit Ain Al-Arab, senior Kurdish official Asya Abdullah told Reuters from inside the town.

“Yesterday there was a violent clash. We have fought hard to keep them out of the town,” she said by telephone. “The clashes are not in the whole of Ain Al-Arab, but in specific areas, on the outskirts and towards the center.”

ISIS, an Al-Qaeda offshoot, has ramped up its offensive in recent days against the mainly Kurdish border town, despite being targeted by US-led coalition led air strikes aimed at halting its progress.

The group wants to take Ain Al-Arab to consolidate a dramatic sweep across northern Iraq and Syria, in the name of an absolutist version of Sunni Islam, that has sent shockwaves through the Middle East.

“There were clashes overnight. Not heavy but ISIS is going forward from the southwest. They have crossed into Ain Al-Arab and control some buildings in the city there,” said Rami Abdulrahman, head of the Syrian Observatory for Human Rights, a group that monitors the conflict with a network on the ground.

“They are about 50 meters inside the southwest of the city,” Abdulrahman said.

An estimated 180,000 people have fled into Turkey from the Ain Al-Arab region following ISIS’s advance. More than 2,000 Syrian Kurds, including women and children, were evacuated from the town after the latest fighting, a member of the Kurdish Democratic Union Party (PYD) said on Monday.

Before the offensive, Ain Al-Arab was home to refugees from the civil war which has pitted rebels against President Bashar Al-Assad and has deteriorated into hundreds of localized battles between different factions.

The most powerful of the myriad militias fighting against Assad, ISIS has boosted its forces with foreign fighters and defectors from other rebel groups. It gained additional heavy weaponry after its fighters swept through northern Iraq in June, seizing arms from the fleeing Iraqi army.

The group released a video showing dozens of men said to be from Ahrar Al-Sham, a rival Islamist group which has clashed with it in the past, pledging allegiance to its leader Abu Bakr Al-Baghdadi, SITE monitoring service said on Monday.

The United States has been bombing ISIS positions in Iraq since August and extended the campaign to Syria in September. Arab states have joined both campaigns, while other Western countries are participating in Iraq but not Syria.

Two months in to the US campaign, the US military has added a new weapon to its arsenal in Iraq, using Apache helicopters for the first time, US officials said on Monday.

Army Major Curtis Kellogg said Baghdad had asked for helicopter support near Fallujah to push back militants west of the capital Baghdad.

The low-flying helicopters give the US military greater capacity to identify individual targets and provide close air support to Iraqi troops in combat, suggesting close cooperation with forces on the ground. But they also expose US troops to far greater risk from ground fire.

Turkey, a NATO member which shares a 500 mile (900 km) border with Syria and has the most powerful military in the area, has so far refrained from joining the campaign, but the plight of Ain Al-Arab has increased pressure to act.

Turkey says the scope of the campaign in Syria should be broadened to seek to remove Assad from power. It has sought a no-fly zone in northern Syria, which would require the coalition to take on Assad’s air force as well as ISIS, a move Washington has not agreed to.

“We are ready to do everything if there is a clear strategy and if we can be sure that our border can be protected after [ISIS is gone],” Prime Minister Ahmet Davutoglu said in an interview with CNN International.

“We don’t want the regime anymore on our border pushing people towards Turkey. We don’t want other terrorist organisations . . .If Assad stays in Damascus with this brutal policy, if [ISIS] goes another radical organisation may come.”

The brother of a British aid worker who was beheaded by ISIS said Britain should put troops on the ground in the Middle East to fight against the militants.

“We need to send ground troops in or forces in to find out where these monsters are and bring them to justice,” said Reg Henning, whose 47-year-old brother Alan was killed last week. “The sooner we do it, the sooner the killing stops.”

“Go and find them, bring them to justice, bring them over here, let us try them,” he was quoted as saying on the BBC website.

Alan Henning’s killing has been condemned by Western leaders and British Muslim groups alike. He was the fourth Western hostage executed by ISIS fighters since the United States launched strikes on the group in Iraq in August.