BOSTON -- U.S. Sen. Ed Markey, D-Mass., on Monday promoted a bill that would limit the U.S. president's ability to use nuclear weapons.

Markey and U.S. Rep. Ted Lieu, D-Calif., have introduced legislation that would prevent the president from launching a first nuclear strike -- one not in response to a nuclear attack -- without a declaration of war by Congress.

"No president should have the power to launch a nuclear first strike without congressional approval," Markey said at a Boston press conference. "Such a strike would be immoral, disproportionate and would expose the U.S. to the threat of devastating nuclear retaliation that could endanger the survival of the American people and human civilization."

Markey introduced the bill amid growing tension between the White House and North Korea. North Korea has been testing missiles it says are capable of hitting the U.S., and has threatened to fire missiles near the U.S. territory of Guam. There is evidence that North Korea has been developing a nuclear missile.

U.S. President Donald Trump said if North Korea threatens the U.S., it will "face fire and fury like the world has never seen." Trump tweeted that the U.S. nuclear arsenal "is now far stronger and more powerful than ever before."

Markey's bill would not limit the president's authority to launch a nuclear strike in response to a nuclear attack. But it would require congressional approval before a president launches a nuclear strike pre-emptively.

Because the Constitution gives Congress the power to declare war, Markey argued, allowing the president to initiate nuclear war unilaterally "is unconstitutional, undemocratic and simply unbelievable." Markey said if enemies of the U.S. believe the U.S. may use nuclear weapons in response to a conventional attack, those countries will feel pressure to build up their nuclear arsenals.

A strong critic of Trump, Markey said the Republican president's inflammatory rhetoric heightens the risk of war. "No human being should have the sole authority to initiate an unprovoked nuclear war, not the American president and certainly not Donald Trump," Markey said.

Markey said he plans to lead a congressional delegation to South Korea, China and Japan in the near future.

Markey held the press conference at the JFK Federal Building in Boston alongside Kathy Robinson, interim director of the Washington office of Women's Action for New Directions, a pro-peace advocacy group that works toward nuclear disarmament and reducing the use of the military.

"It's outrageous any president would have sole authority even when Congress has the constitutional role of declaring war," Robinson said. "It's terrifying when thinking of this president having the authority, who lacks boundaries of appropriate behavior."