Iran said on Tuesday a new U.S.-backed, 30,000-strong force inside Syria would "fan the flames of war", echoing the vehement response of Syria, Turkey and Russia to the plan.
On Sunday, the U.S.-led coalition said it was working with its Syrian militia allies, the mainly Kurdish-led Syrian Democratic Forces (SDF), to set up a force that would operate along the borders with Turkey and Iraq, as well as within Syria.
Syrian President Bashar al-Assad responded by vowing to crush the new force and drive U.S. troops from Syria. Strong Syria ally Russia called the plans a plot to dismember Syria and place part of it under U.S. control, and Turkey described the force as a "terror army."
Foreign Ministry spokesman Bahram Qasemi said such a force would raise tensions in Syria. Iran supports Assad in the nearly seven-year civil war against rebel forces and Islamic State militants, sending weapons and soldiers.
"The U.S. announcement of a new border force in Syria is an obvious interference in the internal affairs of this country," Qasemi was quoted as saying by state news agency IRNA.
Qasemi urged all U.S. forces to leave Syria immediately.
The United States is at the head of an international coalition using air strikes and special forces troops to aid fighters on the ground battling Islamic State militants in Syria since 2014. It has about 2,000 troops on the ground in Syria.