General David Petraeus commanded personnel in Iraq and Afghanistan (Picture: DOD Media).
A former US military chief has said the decision to pull troops out of north-eastern Syria raises concerns about America's "level of commitment" to partners and allies.
General David Petraeus, who commanded forces in Iraq and Afghanistan, spoke to BBC Radio 4's Today programme, criticising the call to leave behind the Kurdish forces who had fought alongside the US against the Islamic State (IS) group.
Gen Petraeus spoke of the remaining threat from IS in the evacuated area, while Mr Trump took to Twitter to warn against an invasion from Turkey.
Turkey are NATO allies to the US, but consider the Kurdish forces terrorists.
Gen Petraeus said: "It certainly does raise concerns about our level of commitment to the partners, who really did the fighting, and in many cases the dying on the ground, to eliminate the ISIS caliphate and to defeat the Islamic State as an army.
"Keep in mind, there are still tens of thousands of Islamic State fighters in groups in north-eastern Syria... very likely trying to establish an insurgent movement as well as to still carry out terrorist attacks."
Although the Syrian Democratic Forces announced earlier this year that they had liberated the last area held by IS, General Petraeus believes the fight could reemerge "if you take your eye off an enemy".
He said: "While the caliphate is defeated, we have to be very careful not to assume that that means that the ISIS problem is ended. It is not."
Referring to his command of coalition forces in Iraq over a decade ago, the General said: "We have seen this movie before... even if that enemy has been destroyed, as we actually did in the surge in Iraq in 2007 to 2008."
The chair of the Foreign Affairs Committee, Tom Tugendhat, said the "surprise withdrawal" of US troops could trigger a humanitarian crisis, urging all parties involved to protect the civilians in the region.
He said the US "will no doubt be mindful of its obligations under international law".
"We also know that where there is a vacuum it will likely be filled by Russia, Iran and its allies," he continued.
"We urge Turkey and the US to work together to establish mechanisms to prevent any further military escalation."