US politicians have voted to authorise training and arming of Syrian rebels to combat Islamist radicals.
The vote is a crucial step in US President Barack Obama's plan to thwart extremism surging across Iraq and Syria.
The House of Representatives voted 273 to 156 to approve Mr Obama's train-and-equip plan.
The approval came despite misgivings by war-weary Democrats that the move could open the door to full-blown US military intervention in the Middle East.
There was also concern among conservatives that the plan falls short of what is needed to defeat the Islamic State group.
The measure was included as an amendment to a stop-gap federal spending measure, which also easily passed the house.
The overall bill now shifts to the Senate, where leaders are confident it will pass today and head to the president for his signature.
Mr Obama has pressed Congress to provide him political cover to initiate military action in Syria against IS.
However, the White House and many politicians believe he has the constitutional authority to launch air strikes in Syria, as he has done in Iraq, to protect US national security interests.
Mr Obama yesterday ruled out a US combat mission in Iraq to help fight IS militants.
Some Democrats warned that the move to train Syrian rebels marked a slippery slope that would lead to US boots on the ground.
But Secretary of State John Kerry, back from a week of international coalition-building in the region, echoed Mr Obama in insisting today's case was different.
"This is not the Gulf War in 1991. It is not the Iraq war in 2003," Kerry argued before the Senate Foreign Relations Committee, a panel he used to chair as a senator.
Elsewhere, UN agencies have said a measles vaccination campaign in northern Syria had been halted after 15 children died following inoculations.
"For as long as the facts remain unclear, the suspension of the immunisation campaign in both Idlib and Deir Ezzour provinces is a wise step," the World Health Organisation and UN children's agency UNICEF said.
