"This resolution will give our armed forces the continuity we need for the next three years. It is not a timetable. It is not announcing that the mission is completed at any given period. What it is saying is that Congress should revisit the issue at the beginning of the next president's term," he said.

Obama said he is "optimistic" that the legislation can "win strong bipartisan support" and show that "Americans are united in this mission."

"In crafting this resolution, we have consulted with and listened to both Republicans and Democrats in Congress," he added.

"We've seen reports of sinking morale among ISIL fighters as they realize the futility of their cause," Obama said, using another acronym for the group. But he also acknowledged that the mission to defeat the IS will remain "difficult for some time."

In a letter accompanying the legislation, Obama warned that if the extremist group was left unchecked, it would "pose a threat beyond the Middle East, including to the United States homeland."