Turkish forces who launched multiple artillery rounds near a US Special Operations outpost in northeastern Syria on Friday have known for months that Americans were there, according to four current and former US officials, raising questions whether Turkey is trying to push American troops farther from the border.

The incident occurred on a hilltop base overlooking the town of Kobane as Turkey continues an operation launched Tuesday against Syrian Kurds, some of whom the United States has partnered with for years in its campaign against the Islamic State. The incursion has focused on an area 100km to the west of Kobane, but US officials believe Turkey has long-term aspirations to control a much larger swath of Syria.

The rounds landed about 9pm within a few hundred yards of the base on Mistenur Hill, US officials said. Navy Capt. Brook DeWalt, a Pentagon spokesman, said in a statement that the US troops "came under artillery fire" but were unharmed and that there was an explosion.

"The US demands that Turkey avoid actions that could result in immediate defensive action," DeWalt said.

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In a statement issued Friday, Turkey's Defense Ministry said its troops had not fired on the Americans and were acting "in self-defense" after one of their border posts was attacked.

But the situation, first reported by Newsweek, was more serious than characterised Friday, several officials said, speaking on the condition of anonymity because of the sensitivity of the issue.

One Army officer who has deployed to northeastern Syria and has knowledge of the situation said that multiple rounds of 155mm fire were launched from Turkey's side of the border and that they had a "bracketing effect" in which shells landed on both sides of the US outpost.

"That's an area weapon," the officer said, referring to the effects the rounds have. "That's not something we ever would have done to a partner force."

The officer said that Turkey knew there were Americans on the hill and that it had to be deliberate. The service members vacated the outpost after the incident but returned on Saturday, according to a US official and images circulating on social media.

"We had been there for months, and it is the most clearly defined position in that entire area," the officer said.

Syrians bury Syrian Democratic Forces fighters killed fighting Turkish advance in the Syrian town of Qamishli yesterday. Photo / AP
Syrians bury Syrian Democratic Forces fighters killed fighting Turkish advance in the Syrian town of Qamishli yesterday. Photo / AP

Brett McGurk, a former special envoy for both the Obama and Trump administrations in the campaign against the Islamic State, raised concerns about the incident on Friday, saying on Twitter that the United States had declared the position to Turkey.

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"This was not a mistake," he said.

McGurk, who often collaborated with the US military in Syria before resigning his position in December, emphasised the increasing risks to Americans throughout Syria in an email Saturday.

"Turkey wants us off the the entire border region to a depth of 30 kilometres," he said. "Based on all the facts available, these were warning fires on a known location, not inadvertent rounds."

Turkey launched its operation into Syria on Tuesday, two days after US President Donald Trump and Turkish President Recep Tayyip Erdogan discussed the issue in a phone call and the White House announced that the United States would not stand in Turkey's way. Trump, explaining his decision, said Monday that he wants to end "endless wars" in the Middle East.

The move immediately raised concerns that the United States was abandoning Syrian Kurds, who have been the closest US partner in counter-terrorism operations against the Islamic State. The Kurdish militia known as the People's Protect Units, or YPG, has formed the backbone of US efforts in northeastern Syria and collaborated with US troops, but Turkey considers the group to be part of a Kurdish movement, the PKK, that it deems a terrorist organisation.

Local residents look on as a convoy of Turkish forces' trucks transporting tanks is driven on a road towards the border with Syria in Sanliurfa province. Photo / AP
Local residents look on as a convoy of Turkish forces' trucks transporting tanks is driven on a road towards the border with Syria in Sanliurfa province. Photo / AP

Turkey said in a statement Friday that it opened fire after Kurdish forces launched rounds at them but stopped when the United States warned that the rounds were too close. US officials confirmed Saturday that the firing ended after US forces reached out to the Turks, but some questioned whether the Kurdish were involved at all.

Defense Secretary Mark Esper and Army Gen. Mark Milley, chairman of the Joint Chiefs of Staff, told reporters Friday that they would not abandon the Kurds but acknowledged that the Pentagon has withdrawn forces from Tal Abyad and Ras al-Ayn in response to Turkey's incursion in between the two border towns.

Turkey appears to have aspirations to push the United States away from Kobane, as well, several officials said. The Army officer with knowledge of Syria said that Turkish forces previously have launched artillery over the border near US forces.

Milley, speaking Friday at the Pentagon shortly before the incident outside Kobane, said the Turkish military "is fully aware, down to explicit grid coordinate detail" of the location of US troops in Syria. He said that he another senior US military officials are coordinating with the Turks "to make sure that they know exactly where American forces are."

Another US defense official, also speaking on condition of anonymity because of the sensitivity of the issue, said that the Pentagon "obviously" told Turkey where US troops are and "they certainly go closer than we would have liked" in the incident outside Kobane.

"Whether that's intentional or reckless, either way it's troubling," the official said.