ERBIL, Iraq- Iraq's Shi'ite militias said on Wednesday that they had killed two Islamic State commanders who ordered an attack last week on U.S.-backed Syrian Kurdish forces along the Iraq-Syria border.

The clashes since last Friday in the border have demonstrated the continuing threat from Islamic State, months after authorities on both sides of the frontier declared the militants had been effectively defeated.

Islamic State fighters, driven from nearly all the territory they once controlled on either side of the Syria-Iraqi border, launched one of this year's fiercest attacks on Friday against the U.S.-backed Syrian Democratic Forces (SDF).

The Syrian Observatory for Human Rights monitoring group said around 70 SDF fighters were killed in the assault, which Islamic State launched under cover of a sandstorm using suicide bombers and female jihadists. The SDF says it lost 14 fighters.

Iraq's Shi'ite paramilitaries, also known as the Popular Mobilization Forces (PMF), said in a statement on Wednesday they had killed two IS commanders in the border area who were responsible for the attack.

Ahmed Nasrallah, a PMF operations commander for western Anbar, said that the Iraqi military had provided information on militant gathering locations, and that a U.S.-led military coalition fighting against Islamic State had not attacked.

The coalition could not immediately be reached for comment.

Islamic State fighters, who once maintained a self-proclaimed caliphate straddling Iraq and Syria, were pushed out of the towns and cities they controlled last year by a range of foes, including the U.S.-led coalition, the Iran-backed Iraqi militia, the Iraqi government and the Moscow-backed Syrian army.

Washington's main allies on the ground in Syria have been the SDF, which is led by a Kurdish militia. The United States has an estimated 2,000 special forces troops on the ground in Syria assisting them.

Iraqi Shi'ite militias reinforced their side of the border area in response to Friday's attack on the SDF in Syria, and Iraq's military said it was ready to take on any militants who tried to cross.

(Reporting by Raya Jalabi) ((Raya.Jalabi@thomsonreuters.com;))