BEIRUT (AP) — Iraqi Shiite forces and Syria accused the United States on Monday of targeting their troops inside Syria with an airstrike, a charge the U.S. denied but that ratcheted up tensions in the area.

Iraq’s Iran-backed Popular Mobilization Forces, the units’ umbrella organization, said in a statement that U.S. aircraft late Sunday night fired two missiles that hit a group of its fighters deployed along the Iraq-Syria border to prevent breaches by the Islamic State group. The statement said the attack left 22 fighters dead and 12 wounded, adding that Iraqi and Syrian authorities were aware of their deployment.

“We demand that the American side issues a clarification for what happened,” the Iraqi group said.

Syrian state media had reported earlier that the airstrike against pro-government forces in the far east of the country had caused casualties, while Iraqi officials said it had killed at least 25 Shiite paramilitaries and was just across the border from its own territory.

Syrian state TV report blamed the attack on the U.S.-led coalition battling the Islamic State group, saying it occurred around midnight in the village of al-Hari, to the southeast of the border town of Boukamal. But a coalition spokesman denied that it had carried out any strikes in the area.

The state TV report, quoting an unnamed military official, gave no breakdown of the casualties other than saying there “were several martyrs and others were wounded.”

In Baghdad, Iraqi officials said state-sanctioned Shiite paramilitaries came under attack south of the town of Qaim, just across the border from Boukamal. They said 25 fighters were killed, three are missing and about 30 were wounded. But did not give details into how the attack was carried out, saying only that investigations were underway, speaking on condition of anonymity because they were not authorized to speak to the media.

The officials said the dead were mostly members of Iraq’s Hezbollah Brigades, which have been active in Syria’s civil war fighting alongside government forces. Also killed were some members of the Sayyed al-Shuhada Battalions, they said.

In Syria, an official with the so-called “Axis of Resistance” led by Iran, which includes Iran, Syria, Hezbollah and other groups fighting alongside President Bashar Assad’s forces, told The Associated Press that the attack on Syrian and Iraqi positions on both sides of the border had been carried out by American drone aircraft.

The official, who spoke on condition of anonymity because of the secrecy of information, added that the troops had been part of a push against IS extremists in the area.

U.S. military spokesman Col. Sean Ryan said the coalition was looking into the reports.

“We are aware of the strike near Boukamal, however there have been no strikes by U.S. or coalition forces in that area,” he said. “We’re looking into who that could possibly be, but it wasn’t the U.S. or the coalition.”

The Britain-based Syrian Observatory for Human Rights, a war monitor, said the airstrike killed 52 foreign fighters allied with the Syrian government, mostly Iraqis. Shiite militias fighting alongside government forces in Syria include large numbers of Iraqi, Lebanese and Afghan fighters.

Last week, IS launched a major offensive against Boukamal, reaching the outskirts of the town before being pushed back by government forces. The loss of the town would deal a major blow to Iran-backed forces on both sides of the border, who have established a corridor through eastern Syria to link Iran to the Mediterranean Sea.

Syrian and Iraqi forces have driven IS from virtually all the territory it once held in both countries, but the militants still control some remote areas along the border.

Syrian troops and allied militias, backed by Russian airstrikes, have been conducting operations west of the Euphrates River, while the U.S.-backed Syrian Democratic Forces, a Kurdish-led militia, is operating on the eastern banks. The U.S.-led coalition has struck pro-government forces in the past when they have tried to cross the river. The overnight attacks took place on the western side.

Meanwhile in the country’s north, a deal reached between Turkey and the U.S. to have Kurdish forces withdraw from a town appears to have gone into effect.

Turkey announced its troops began patrols on the outskirts of the key northern Syrian town of Manbij following a recent deal struck with the United States. The army tweeted Turkish and American troops began patrols Monday along the outskirts of Manbij and an area controlled by Turkey-backed forces.

It said the move was part of the Turkish-U.S. deal reached in early June, aiming to secure the town and push out a Syrian Kurdish militia.

President Recep Tayyip Erdogan confirmed that the patrols had begun in a speech Monday in northern Samsun province.

Manbij has been a source of tension between the two NATO allies after an American-backed Syrian force took the town from the Islamic State group in 2016.

The Syrian Kurdish YPG forms the backbone of that force but Turkey considers the group terrorists and has been demanding the U.S. cease its support. YPG announced it would leave the strategic town.