Trump said in April he wanted to withdraw American troops from Syria relatively soon, but also voiced a desire to leave a "strong and lasting footprint". "The maintenance of that deconfliction zone is important and we would view very gravely any actions that tended to change that".
"Any interested party in Syria should understand that attacking USA forces or our coalition partners will be a bad policy", Director of Joint Staff Lieutenant General Kenneth McKenzie said at the Pentagon press briefing on Thursday.
In an interview with Russia's RT global broadcaster, Assad said he would negotiate with fighters backed on the ground by Washington, but would reclaim territory they control by force if necessary, whether or not American troops supported them.
Assad's threat to use force against USA allies in Syria and about 2,000 American troops providing them with air support and training prompted a warning from the Pentagon.
"The Americans should leave somehow".
In the RT interview, Assad responded sharply to U.S. President Donald Trump's recent description of him as an "animal", saying, "What you say is what you are".More news: Sharapova overwhelms Pliskova at French Open
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President Assad was speaking just over a week after his government declared it was in full control of the suburbs around Damascus for the first time in seven years.
Israel has warned it will not tolerate having Iranian advisers and the Shiite militias they support move close to its border.
Mounting concerns that escalating Israeli strikes against Iranian targets in Syria would result in direct clashes with Russian forces operating in the war-torn country were apparently put to bed on Thursday. As part of the deal, Russian Federation is also demanding the withdrawal of all foreign forces - including Iranian, American and Turkish forces - from Syrian territory.
Backed by Russian air power and Iranian and Hizbullah militias on the ground, Assad's forces have gained significant ground in recent months in the seven-year civil war that has killed an estimated half a million people and driven another 5 million overseas as refugees.
Mr Assad declined to say how much longer the war will continue but said he intends to bring all parts of the country back under his control.
Every attempt would be made to negotiate with the SDF, he told RT, but if negotiations fail, "the Syrian army will be forced to liberate areas occupied by the SDF, with the Americans or without the Americans".