A US military outpost in Syria came under fire as Turkey ramped up its air and artillery strikes in north-east Syria on Friday, escalating its offensive against the Kurds amid warnings that the operation could lead to the revival of ISIS in the region.
Turkey was forced to issue a statement denying it had deliberately targeted a US observation post near Kobane on the Syrian border after reports emerged of an unexplained explosion near the base.
"Turkey did not open fire at the US observation post in any way," the statement said, explaining that Turkish forces had retaliated in self-defence after its own base, only 1000 metres from the US outpost, was targeted by Kurdish mortar fire.
The Chairman of the Joint Chiefs of Staff General Mark Milley told reporters at the Pentagon earlier on Friday that Turkey had been told of US positions in Syria, "down to explicit grid coordinate detail."
Top Pentagon officials stressed the need for Turkey to avoid doing anything to endanger US forces inside Syria, which numbered about 1,000 before the incursion.
The escalation came as the Kurds, who recaptured swathes of northeastern Syria from ISIS with the backing of the United States, say the Turkish assault could allow the extremist group to re-emerge.
In its first big attack since the assault began, ISIS claimed responsibility for a deadly car bomb in Qamishli, the largest city in the Kurdish-held area, even as the city came under heavy Turkish shelling.
Five ISIS fighters fled a jail there, and foreign women from the group being held in a camp torched tents and attacked guards with sticks and stones, the Kurds said.
Earlier on Friday, Russian President Vladimir Putin said Turkey's Syria incursion could lead to the revival of ISIS in the region.
Mr Putin said the Syrian Kurdish militias who were guarding thousands of imprisoned ISIS fighters were now fleeing after Turkey launched artillery and air strikes on Wednesday.
"I'm not sure whether the Turkish army will be able to take this under control – and how soon," the Russian president told a meeting of heads of the Commonwealth of Independent States (CIS), an association of ex-Soviet countries, in Turkmenistan.
According to the Kurdish administration, about 12,000 ISIS prisoners are being held in seven detention centres across Kurdish-controlled areas.
The possible escape of thousands of extremist fighters "is a real threat to us", Mr Putin said.
"How will they be moving and to where? Through Turkish territory? Through other territories?" he asked.
"We should simply understand this, know and mobilise the resources of our security services to neutralise this emerging new threat."
The Kurdish forces targeted by Turkey were the US-led coalition's main ground partner in years of battle against ISIS and its now-defunct "caliphate".
Nato chief Jens Stoltenberg also raised concerns about a resurgence of ISIS after a meeting with Turkish Foreign Minister Mevlut Cavusoglu in Istanbul.
“An imminent concern is that captured Daesh terrorists must not be able to escape,” the Nato secretary general said at a press conference with Mr Cavusoglu.
US President Donald Trump, whose order to pull back US troops from the border this week effectively triggered the invasion, said Washington would now seek to broker a truce.
Turkey's offensive, its third incursion since the start of the war in Syria, was met with fierce international condemnation, including from some of Mr Trump's own allies, over what many saw as the blatant betrayal of a faithful ally and over fears of another humanitarian crisis.
Turkish and allied forces faced stiff Kurdish resistance on Friday as they battled to seize key Syrian towns along a roughly 120 kilometre long segment of the border.
"There is heavy fighting between the SDF and the Turks on different fronts, mostly from Tel Abyad to Ras Al Ain," the Syrian Observatory for Human Rights said.
The Britain-based war monitor said the Turkish forces and their Syrian proxies – mostly Sunni Arab former rebels – were deploying air strikes, heavy artillery and rocket fire.
"The SDF are using tunnels, trenches and berms" in their defence operations, the Observatory said.
Kurdish counter-attacks overnight led to the retaking of two of the 11 villages they had lost since the start of the Turkish-led assault on Wednesday.
The Observatory and a Kurdish military source said that several Arab families in the border area had sided with Turkey, raising sleeper cells to attack from behind SDF lines.
The war monitor said at least 10 civilians and 29 SDF fighters had been killed since the launch of the offensive on Wednesday. The Turkish military announced its first fatality on Friday.
Ras Al Ain, Tal Abyad and other border towns between them have been almost emptied of their population in a huge wave of displacement.
Most of the 70,000 people the United Nations confirmed had been displaced travelled east towards the city of Hassakeh, which has not been targeted by Turkey.
Syrian Kurdish officials said on Friday they were evacuating the Mabrouka camp for displaced people after several Turkish artillery shells landed nearby. The camp lies about 12km from the border and is home to about 7,000 people who fled ISIS.
Turkish President Recep Tayyip Erdogan wants to create a buffer between the border and territory controlled by Syrian Kurdish forces, who have links with Turkey's own Kurdish rebels.
He also plans to use the strip, which he envisions will be about 30 kilometres deep and is mostly Arab, as an area in which to send back some of the 3.6 million Syrian refugees who live on Turkish soil.
The area would be under Turkish control and run by Syrian proxies, a move that would make it hard for displaced Kurds to return and would durably reshape the area's ethnic map.
On Twitter on Thursday, Mr Trump said that he hoped to "mediate a deal between Turkey and the Kurds" – saying the alternatives were sending in "thousands of troops" or hitting Turkey hard with sanctions.
A US official explained that Mr Trump had asked diplomats to try to broker a ceasefire agreement and argued sanctions against Turkey were not justified at this stage.
Asked to define what actions would violate Mr Trump's vague warning, the official said they would include "ethnic cleansing … indiscriminate artillery, air and other fires directed at civilian populations".
Aid groups have warned of yet another humanitarian disaster in Syria's eight-year-old war if the offensive was not stopped.
France, which was the United States' top partner in the anti-ISIS coalition, has threatened sanctions against fellow Nato member Turkey. A French official said sanctions could be discussed at next week’s European Union summit.
European Union Council chief Donald Tusk said on Friday that Turkey’s military operation was of “grave concern” and urged Ankara to stop its military incursion before it triggers another “humanitarian catastrophe.”
He said Turkey’s security concerns should be dealt with through diplomatic and political means and that military action only exacerbated civilian suffering, caused further displacement of people and threatened the progress made in battling ISIS.
The Syrian Kurdish forces have been “crucial” in fighting ISIS and abandoning them “is not only a bad idea” but raises many “questions both of a strategic and moral nature”, Mr Tusk said.
He also strongly criticised Mr Erdogan for suggesting he would send 3.6 million Syrian refugees living in Turkey to Europe unless the 28-member bloc stopped calling Turkey’s action an “invasion”.
Mr Erdogan’s remarks were “totally out of place”, he said, adding that the EU would never accept “that refugees are weaponised and used to blackmail us”.