Iran and Syria on Monday demanded the full withdrawal of American forces in Syria and threatened to attack US-backed Kurdish forces if they did not return areas under their control to state authority.
The demand was made by the Iranian and Syrian military chiefs after a meeting in Damascus attended by their Iraqi counterpart. Iraq's defence ministry said the meeting was to strengthen security co-operation.
The US has said it will withdraw most but not all of its estimated 2,000 troops in Syria after the defeat of ISIS, which is clinging to its last piece of territory against an offensive by the Syrian Democratic Forces. The Kurdish-dominated SDF now controls large areas of northern Syria retaken from ISIS.
"The only card remaining in the hands of the Americans and their allies is the SDF, and it will be dealt with through the two methods used by the Syrian state: national reconciliation or the liberation of the areas that they control through force," Syrian Defence Minister General Ali Abdullah Ayoub said.
The Iranian army chief of staff, Major General Mohammad Bagheri, who arrived in Damascus on Sunday, said the meeting had "studied the means that should be taken to recover" territories still outside Syrian government hands, including the areas of US deployment.
Gen Bagheri is expected to visit Iranian troops in the Eastern Ghouta region near Damascus and Deir Ezzor province, where ISIS is making its last stand in the village of Baghouz near the Iraq border.
Iran has been one of the most dedicated supporters of Mr Al Assad during the uprising against his regime that began in 2011 and is also a close ally of Iraq.
President Hassan Rouhani completed a three-day visit to Iraq last week, sending a strong message to the US that Tehran still dominates Baghdad, according to observers.
Washington stressed that it wants to contain Tehran’s “destabilising influence” in the region, including in Iraq and Syria.
The US imposed economic sanctions on Tehran and withdrew from the 2015 Iranian nuclear deal.
Its military has a presence in eastern Syria to support the Kurdish-led Syrian Democratic Forces who are fighting ISIS.
In south-eastern Syria, the US military base Al Tanf is near the Damascus-Baghdad highway.
The Iraqi military chief, Lt Gen Othman Al Ghanmi, said Baghdad and Damascus hoped to soon reopen their border which has been closed for years after ISIS overran large areas of Iraq and Syria in 2014.
"God willing the coming days will witness the opening of the border crossing and the continuation of visits and trade between the two countries," Lt Gen Al Ghanmi said at a news conference broadcast by Syrian state television.
A similar meeting was held in Baghdad with Russian, Iranian and Syrian officials last December.