Sources in Syria and Iraq reported Tuesday that at least 40 militiamen from a powerful Iraqi Shiite militia were killed in a U.S.-led coalition attack across the border with Syria.

The deputy head of the Iranian-backed militia known as Kattaib Sayeed al-Shuhadaa blames U.S. forces for the assault. The spokesman of the U.S.-led coalition, Army Col. Ryan Dillon, dismissed the allegation.

The militia's Ahmed al-Maksousi says U.S. forces hit their base in Syria's Jamouna area with artillery early on Monday. He claims that was followed by an on-the-ground attack by the Islamic State group.

Sky News in Arabic quoted sources in the field who reported that it was an airstrike that hit the sites, saying it targeted a company of Al-Hasd al-Shaabi fighters, south of Sinjar. The area is about 12 kilometers — around 7.5 miles — from the Iraqi border.

Al-Maksousi says that along with 40 killed, at least 30 militiamen were wounded.

The Iran-supported militia is also taking part in the fight against the Islamic State in Iraq and supports the Iraqi government forces. The group's name has often been linked to attacks against Sunni civilians.

Competition between the Syrian army and militias and the U.S.-backed Free Syrian Army (FSA) rebels has stepped up in the region near the Syria-Iraq border. In June, U.S. forces shot down an armed, Iranian drone bearing down on its forces in the area.