An Army captain sued President Barack Obama on Wednesday, alleging that he doesn't have the proper congressional authority to wage war against Islamic State militants in Iraq and Syria.

Capt. Nathan Michael Smith filed the suit in US District Court in Washington as the president is deploying more special operations forces to the region — and a day after a Navy SEAL was killed in combat in Iraq, the third since a US-led coalition launched its campaign against the Islamic State in the summer of 2014.

Smith supports the war on military and moral grounds and considers the Islamic State an "army of butchers." But he wants the court to tell Obama that he needs to ask Congress for a new Authorization for the Use of Military Force.

The White House did not comment on the lawsuit.

To fight IS, Obama has been relying on congressional authorizations given to President George W Bush for the war on al-Qaida and the invasion of Iraq. Critics say the White House's use of post-9/11 congressional authorizations is a legal stretch at best.

The White House has claimed it has all the authority it needs to wage the war against IS, but says if an authorization tailored specifically for IS passed Congress with bipartisan support, it would send a clear signal of unity to US troops and those groups they are fighting.

Several lawmakers have pushed for a new authorization and the White House sent its own version to Capitol Hill. But many lawmakers have no interest in casting a war vote, leaving the issue languishing in Congress.

Smith is asking the court to find that the war against IS violates the War Powers Resolution because Congress has not declared war or given the president specific authorization to fight it.

"This lawlessness has made it impossible for Capt Smith to determine whether his present mission is inconsistent with his oath to 'preserve, protect and defend the Constitution of the United States,' thus requiring him to seek an independent determination of this matter from the court," the suit said.

Members of the military are obligated to refuse to follow an order that is illegal under the Uniform Code of Military Justice. If they follow unlawful ones, they risk punishment.