U.S. President Barack Obama on Wednesday sent Congress his long-awaited formal request to authorize military force against Islamic State, meeting swift resistance from Republicans as well as his fellow Democrats wary of another war in the Middle East.

Republicans, who control Congress and say Obama's foreign policy is too passive, want stronger measures against the militants than outlined in the plan, which bars any large-scale invasion by U.S. ground troops and covers the next three years.

Obama acknowledged that the military campaign is difficult and will remain so. "But our coalition is on the offensive. ISIL is on the defensive, and ISIL is going to lose," he said in a televised statement from the White House.