AKCAKALE - Turkish forces faced fierce resistance from US-allied Syrian Kurdish fighters on the third day of Ankara’s offensive in northern Syria as casualties mounted, international criticism of the campaign intensified and estimates put the number of those who fled the violence at 100,000. In a complicating twist, Washington said its troops also came under fire from NATO ally Turkey.

No US troops were hurt in Friday’s explosion at the small US outpost, and the artillery strike marked the first time a coalition base was in the line of fire since Turkey’s offensive began.

US officials said the Americans have vacated the post on a hill outside the town of Kobane, and added that a large base in the town was not affected by the shelling. The officials spoke anonymously because they were discussing an ongoing military operation.

Turkey said the US was not targeted and its forces were returning fire from Kurdish fighters about half a mile from the US outpost. The Turkish Defence Ministry said it ended the strike after communicating with the US.

Navy Capt Brook DeWalt, a Pentagon spokesman, says the explosion came within a few hundred metres of the area where US troops were.

No US troops hurt in Friday’s explosion at small outpost

The artillery strike so close to American forces showed the unpredictable nature of the conflict days after US President Donald Trump cleared the way for Turkey’s air and ground invasion, pulling back US forces from the area and saying he wanted to stop getting involved with “endless wars.”

The decision drew swift bipartisan criticism that he was endangering regional stability and risking the lives of Syrian Kurdish allies who brought down the Islamic State group in Syria.

Earlier, Turkey said it captured more Kurdish-held villages in the border region, while a hospital in a Syrian town was abandoned and a camp of 4,000 displaced residents about 12 kilometres from the frontier was evacuated after artillery shells landed nearby.

Reflecting international fears that Turkey’s offensive could revive the Islamic State group, two car bombs exploded outside a restaurant in the Kurdish-controlled urban centre of Qamishli, killing three people, and the extremists claimed responsibility. The city also was heavily shelled by Turkish forces.

Turkish shelling hit a prison holding IS militants in Qamishli, Syrian Kurdish officials said. They shared a video Friday showing a shell landing in the courtyard of what appears to be a prison facility. Seconds later, a handful of men open doors and seem to be trying to leave.

Kurdish fighters waged intense battles against advancing Turkish troops that sought to take control of two major towns along the Turkish-Syrian border, a war monitor said.

The UN estimated the number of displaced at 100,000 since Wednesday, saying that markets, schools and clinics also were closed. Aid agencies have warned of a humanitarian crisis, with nearly a half-million people at risk in northeastern Syria.

Turkish President Recep Tayyip Erdogan said his country “will not take a step back” from its offensive. “We will never stop this step. We will not stop no matter what anyone says,” he said in a speech Friday. Turkey’s military said it captured a key Syrian border town under heavy bombardment Saturday as its offensive against Kurdish fighters pressed into its fourth day.

Turkish troops entered central Ras al-Ayn according to Turkey’s Defence Ministry and a war monitor group, marking the most significant gain since the invasion began Wednesday. The ministry tweeted: “Ras al-Ayn’s residential center has been taken under control through the successful operations in the east of Euphrates” river.

An Associated Press journalist across the border in Turkey heard the sound of sporadic clashes as Turkish howitzers struck the town and Turkish jets screeched overhead.

Syrian Kurdish forces appeared to be holding out in some areas of the town. The Kurdish Syrian Democratic Forces released two videos said to be from inside Ras al-Ayn, showing fighters saying that it is Saturday and they are still there.

The fighting was ongoing as the Kurdish fighters sought to reverse the Turkish advance into Ras Al-Ayn, the Syrian Observatory for Human Rights said.

Earlier in the day, Turkish troops moved to seize control of key highways in northeastern Syria, the Turkish military and the Observatory said. Turkey’s state-run Anadolu news agency said that Turkey-backed Syrian opposition forces had taken control of the M-4 highway that connects the towns of Manbij and Qamishli.

The SDF said that Turkish troops and their Syrian allies reached the highway briefly before being pushed back again.

Turkish troops also cut the route linking the northeastern city of Hassakeh with Aleppo, Syria’s largest city and once commercial center, according to the Observatory.

Since Wednesday, Turkish troops and Syrian opposition fighters backed by Ankara have been advancing under the cover of airstrikes and artillery shelling, reaching the Manbij-Qamishli road about 30 kilometers (19 miles) south of the Turkish border.

Turkey has said it aims to push back the Syrian Kurdish People’s Protection Units, or YPG, which it considers terrorists for its links to a decades-long Kurdish insurgency within its own borders. The YPG is a main component of the SDF.