US Special Forces have been "mistakenly" bombed by Turkey in the northern Syrian town of Kobani.
The Pentagon confirmed US troops came under artillery fire from the Turks on Friday, in an area where Turkey knows Americans are present.
Navy Captain Brook DeWalt, a Pentagon spokesman, said the artillery explosion came within a few hundred metres of the area where US troops were.
"Turkish artillery hit close to a US special operations unit near Kobani on Friday," DeWalt confirmed.
"The artillery shells hit several hundred metres from where the US special ops forces were located. There are no US injuries."
The shelling occurred on Mashtenour hill in Kobani, a border city under Kurdish control. Turkish forces are moving in on the region to reclaim it from the YPG, a group of Kurdish fighters.
Turkish forces allege that YPG fighters are planning to launch attacks over the border from the area.
US President Donald Trump, when asked about the shelling on the White House lawn on Friday afternoon, said: "We'll take a look at it".
It wants to move in and create a "safe zone", wiping out the Kurds and any ISIS prisoners being kept there.
Earlier this week, Trump agreed to remove US Special Operation Units which have been in the region for years, bolstering the Kurds in their fight against ISIS, and the Syrian Army.
Newsweek cited anonymous Department of Defense officials who said it was unlikely that the Turkish army would mistake US troops for Kurds in that case and that they would be aware of US positions "down to the grid".
An Iraqi Kurdish intelligence official confirmed the shelling.
CNN reported that there was no indication the shelling was deliberate.
Turkey's military also issued a statement insisting it was an accident and that it was returning fire on the Kurds.
It said that as soon as it realised US Special Forces had potentially been caught in the middle, it ceased.
However journalists in the region cite other officials who say it is impossible it was a mistake.
"They know we are there, we told them our position. There's no other target in the area. They're trying to drive us out. If Turkey can get us to leave so they can siege Kobane, it's all over," one Coalition officer told Telegraph reporter Josie Ensor.
The unnamed official said the shelling was so heavy, US forces considered returning fire.
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It comes after President Trump agreed to remove US troops from the region, where they have been bolstering the Kurds since the start of the Syrian civil war.
Turkey is fighting them, claiming they are a terrorist organisation.
Trump has been criticised for the decision, with some fearing it could enable ISIS fighters in the region by removing one of the key groups fighting them, and expose the Kurds to harm.
He has insisted he will impose sanctions on Turkey if it acts irresponsibly.
US Treasury Secretary Steve Mnuchin also fired a warning shot at Turkey on Friday, saying Trump is prepared to impose "very significant" sanctions on Ankara if President Recep Tayyip Erdogan steps out of line.
Mnuchin said that Trump had signed an executive order giving Treasury the authority, in consultation with the State Department, to impose sanctions a moment's notice. He declined to elaborate on potential targets but said financial institutions had been put on notice.
Turkey has also been told to "not allow even a single ISIS fighter to escape" from captivity, he announced from the White House podium at a short-notice briefing.
Despite the warning, he said Erdogan's invitation to visit the White House on November 13 remained intact.
- additional reporting Daily Mail