US President Barack Obama has been sued by an army captain, who claims that the war against ISIS is illegal. It lacks congressional authorization.

Capt. Nathan Michael Smith, 28, who is currently serving in Kuwait, said that the military stance explicitly violates the 1973 War Powers Act.

“Under the War Powers Act, every time [a president] notifies leaders of military strikes, his power to carry them out without congressional approval extends another 60 days,” reported NPR.

Smith filed the lawsuit in U.S. District Court for the District of Columbia Wednesday.

As per the court documents, Smith says that fighting an illegal war is against his oath to ‘preserve, protect, and defend’ the Constitution.

In his lawsuit, he included: “ The President did not get Congress’s approval for his war against ISIS in Iraq or Syria within the sixty days, but he also did not terminate the war. The war is, therefore, illegal. The Court should issue a declaration that the War Powers Resolution requires the President to obtain a declaration of war or specific authorization from Congress within sixty days of the judgment, and that his failure to do so will require the disengagement, within thirty days, of all United States armed forces from the war against ISIS in Iraq and Syria.”

However, Defense Secretary Ash Carter and other officials from Obama administration said that they have legal authorisation to carry out fight against Islamic State group, reports Army Times.

According to the lawsuit, the president has surpassed the authority laws under 2001 and 2002 AUMFs. It claims that ISIS had no role in the attacks of Sept. 11, 2001.

The anti-terrorism campaign against ISIS in Syria ranges from airstrikes to deployed army personnel.

On Tuesday, a US Navy Seal was killed in an intense fight between US special forces, Kurdish commandoes, and Islamic State fighters.

And as the campaign gets more fierce, there has been no formal military force authorization from Congress.