U.S. President Barack Obama speaks about the Supreme Court to students and faculty at the University of Chicago Law School, where Obama taught constitutional law for over a decade, in Chicago, April 7, 2016. REUTERS/Kevin Lamarque

A U.S. Army captain filed a lawsuit against President Barack Obama Wednesday, alleging that Obama does not have the authority from Congress to wage the fight against the Islamic State group in Iraq and Syria. The suit comes after an American serviceman became the third soldier to be killed in Iraq since U.S. troops pulled out from the country in 2011.

Capt. Nathan Michael Smith, an intelligence officer deployed in Kuwait, filed the suit in U.S. District Court in Washington. The 28-year-old said he supports the fight against the extremist group on military and moral grounds and considers the militants an “army of butchers,” according to court documents shared by the New York Times. But, the captain also wants the court to tell the president that he needs to ask Congress for a new Authorization for the Use of Military Force.

“Under the 1973 War Powers Resolution, when the President introduces United States armed forces into hostilities ... he must either get approval from Congress within sixty day to continue the operation, in the form of a declaration of war ... [the] President did not get Congress’s approval for his war against ISIS [Islamic State group] in Iraq or Syria ... the war is therefore illegal,” Smith’s court documents read.

Smith has urged the court to say that the war against ISIS goes against the War Powers Resolution because the Congress has not declared war or given the president an authorization to fight the militant group.

“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.

The White House has not commented on the legal challenge.

Members of the military are required to refuse to follow an order that is illegal under the Uniform Code of Military Justice. The military members can be punished if they follow unlawful orders.

In order to fight against the Sunni militant group, Obama has been depending on congressional authorizations given to then-president George W. Bush for the war on al Qaeda and the 2003 Iraq invasion. However, critics have maintained that the White House is stretching the use of post-9/11 congressional authorizations, the Associated Press reported.