The US Congress might not vote until next year on an authorization for President Barack Obama's air strikes against Islamic State militants in Iraq and Syria, congressional aides said on Monday, despite some lawmakers' insistence that approval is already overdue.The US military began a campaign in Iraq weeks ago to halt advances by the Sunni group known as Islamic State, which has killed thousands of people while seizing swaths of territory in Iraq and Syria. The United States and five Arab nations began air strikes against targets in Syria last week.Obama has said he does not need approval for the air strikes, despite the constitutional requirement that Congress authorize military action.Many lawmakers disagree. While supporting efforts to stop the group, some have called for Congress to return early from its current recess in the run-up to Nov. 4 elections to debate and vote on what is called an AUMF, authorization for the use of military force.But others argued that an AUMF should not be rushed through in the last weeks of the current Congress. Aides said there is a good chance that view will prevail as lawmakers deal with a host of other issues, such as a long-term spending plan, during what is expected to be a contentious year-end session.Representative Buck McKeon, chairman of the House Armed Services Committee, said an authorization vote should be left to newly elected and re-elected lawmakers. "Incoming representatives will oversee this conflict, and they should bear the responsibility for authorizing it - even if that means a vote can't take place until January," the California Republican said in a letter to The Washington Post on Monday.Lawmakers left Washington in mid-September to campaign for elections in which every seat in the House of Representatives and about a third of the US Senate are up for grabs. They will return in mid-November.Washington Representative Adam Smith, the top Democrat on the House Armed Services Committee, said the issue was less timing than the need to draft an acceptable AUMF."You can write a sentence and have one person look at it and say, 'That gives the president too much power,' and have another person look at the exact same sentence and say that it confines the president too much," Smith said."Getting the language right on an AUMF... that authorizes the president to do what we would like him to do, but no more, is an extraordinarily difficult drafting challenge," Smith said.The cost of the air strikes so far is nearly $1 billion and is likely to run between $2.4 billion and $3.8 billion per year if air and ground operations continue at the current pace, an analysis by the nonpartisan Center for Strategic and Budgetary Assessments said.