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

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

“The U.S. demands that Turkey avoid actions that could result in immediate defensive action,” DeWalt said.

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 characterized 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 U.S. outpost.

That’s an area weapon," the officer said, noting its explosive effects. “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 U.S. 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.

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.

“This was not a mistake,” he said.

McGurk, who often collaborated with the U.S. military in Syria before resigning his position in December, emphasized 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 kilometers,” 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 President 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 U.S. 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 U.S. efforts in northeastern Syria and collaborated with U.S. troops, but Turkey considers the group to be part of a Kurdish movement, the PKK, that it deems a terrorist organization.

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. U.S. officials confirmed Saturday that the firing ended after U.S. forces reached out to the Turks, but some questioned whether the Kurdish were involved at all.

Defense Secretary Mark T. Esper and Army Gen. Mark A. 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 U.S. 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 U.S. troops in Syria. He said that he another senior U.S. military officials are coordinating with the Turks “to make sure that they know exactly where American forces are.”

Another U.S. defense official, also speaking on condition of anonymity because of the sensitivity of the issue, said that the Pentagon “obviously” told Turkey where U.S. 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.