A senior US official told reporters at a security conference in Munich a seven-day reduction of violence agreement had yet to begin.
Turkey's Defence Minister Hulusi Akar and his US counterpart Mark Esper have met in Brussels where they agreed to take concrete steps in Syria.
Trump's salvo came as Iran's Islamic regime faced a challenge from angry street protests, having come to the brink of war with the US after a series of tit-for-tat confrontations.
The non-binding resolution was introduced by Democrats after Trump's order to kill an Iranian commander and retaliatory missile strikes by Iran dramatically escalated tensions between the two foes.
Hashd al Shaabi militiamen and their supporters withdraw from US embassy perimeter in Baghdad, a day after they stormed the premises, forcing Washington to dispatch extra troops and threaten reprisals against Iran.
Pentagon chief Mark Esper's statement comes days after Turkish President Recep Tayyip Erdogan's warned that Ankara may close Incirlik and Kurecik bases where US soldiers are stationed "if necessary".
Rocket strikes targeting Iraqi bases where members of the US-led coalition are also stationed have increased in past weeks with no claim of responsibility from any party.
"I feel confident that we could reduce our numbers in Afghanistan and still ensure that place doesn't become a safe haven for terrorists who could attack the US," Esper said.
Mark Esper and his South Korean counterpart announced that US and South Korea are postponing a joint military air exercise that North Korea has criticised as provocative.
Experts and observers warn US President Donald Trump's desire to keep Syrian oil could promote the view that the US involvement in the Middle East is to extort revenue.
To help Iraq in tackling Daesh, the US offered to move its troops to Iraq after the Syria pullout. But Washington faced an unprecedented denial from Baghdad.
US Defense Secretary Mark Esper arrived in Afghanistan where stalled peace talks with the Taliban and persistent violent attacks by the insurgent group and Daesh militants have complicated the Trump administration’s pledge to withdraw US troops.
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