The American Civil Liberties Union (ACLU) warned President Trump on Tuesday that only Congress can authorize a military strike on North Korea, as tensions with Pyongyang continue to escalate.

“The American Civil Liberties Union strongly urges you to uphold the Constitution, and the fundamental principle of separation of powers, by recognizing the sole authority of Congress to declare war — and to refrain from the use of force against North Korea in the absence of explicit congressional authorization for the use of force,” the group wrote in a letter to Trump on Tuesday.

“This drumbeat of threats over the past several weeks has raised concerns across the country about whether you will abide by the check on the president’s power that is fundamental to the Constitution’s separation of powers on war authority.”

ADVERTISEMENT

The letter cites a number of comments Trump has made over the past month about North Korea after the country test-fired intercontinental ballistic missiles and carried out its sixth nuclear test.

On Sunday, the most recent nuclear test proved to be its most powerful to date. Pyongyang claimed the detonation successfully tested a hydrogen bomb that could be loaded onto an intercontinental ballistic missile.

After the test, Trump said “we’ll see” when asked whether the United States would attack North Korea.

Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE also warned Sunday that any threats to the United States or its allies would be met with a “massive military response.”

In August, Trump threatened to bring “fire and fury” upon North Korea and tweeted that military options were “locked and loaded.”

ADVERTISEMENT

In its letter, the ACLU, which said it “does not take a position on whether military force should be used against North Korea,” highlighted the constitutional authority delegated to Congress to declare war, along with quotes from several framers about the congressionasl role in declaring war.

The group also argued that military action against North Korea would violate international law absent a U.N. Security Council resolution authorizing it.

“The Congress, as representatives of the American citizenry, has exclusive authority under the Constitution to decide whether the president may use military force,” the letter concluded. “Particularly in the wake of your recent threats of military action against North Korea — with allusions to the possible use of nuclear weapons against another country for the first time since World War II — we urge you to make clear that you will refrain from use of force outside the scope of the Constitution and the law.”