RIYADH - Syria's chief opposition negotiator said the United States cannot afford to leave Syria as it has yet to achieve any of its goals in the region, even though President Donald Trump said recently Washington would withdraw its troops."I personally think the US is not capable of withdrawing its fighters from Syria," Nasr Hariri told Reuters on Friday.Washington for years supported rebels militarily against Syrian President Bashar Assad, but ended its train-and-equip program last year after changing its focus to the fight against Islamic State.It helped an alliance of Kurdish and Arab militias drive the jihadists from swathes of northern and eastern Syria last year, including the group's Syrian capital of Raqqa, and has deployed about 2,000 US troops in the country.Trump said this month he wanted to bring them home soon but later agreed they should stay a little longer after his advisers argued they were needed to stop Islamic State re-emerging and to prevent Iran gaining a bigger foothold.The US led limited air strikes against the Syrian government along with Britain and France on April 14 in retaliation for a suspected chemical weapons attack, which Assad denies."Daesh is not finished," Hariri said, using an Arabic acronym for Islamic State. "If we don’t treat the reasons that birthed Daesh, then these would be temporary victories like shifting sands that disappear here and pop up somewhere else. And fighting Daesh is at the top of American priorities."The only way to end the Syrian crisis is by reaching a political solution that replaces Assad because he is only interested in military solutions, said Hariri. But there can only be a political solution if the US and Russia have serious resolve to reach one, he said."It needs an international consensus that begins with a US-Russian agreement," he said.Russia's entry into the Syrian war in 2015 turned the tide in Assad's favor, but Hariri said Moscow would struggle to restore the government's pre-war power."Russia will not be able to take military control of Syrian lands, and the Syrian situation is much more complex than expanding military influence or achieving military gains," Hariri said.Last week, Saudi Arabia's Foreign Minister Adel al-Jubeir said the kingdom would be prepared to send troops into Syria as part of the US-led coalition if a decision was taken to widen it.Hariri said he believes Saudi Arabia and other Arab countries still want a political solution and had stopped giving military support to the rebels because of US policy."Military support stopped last year, not because of a Saudi, Turkish, or Jordanian decision," he said. "(It) happened because of an international decision, and when I say international decision I mean a decision by the United States." "We know, and the Syrian people know, that when the US seriously wants to reach a political solution and put real weight against the table of negotiations, it can make a change," Hariri said.