Kirtland Air Force Base (United States) (AFP) - US Defense Secretary Ashton Carter said Tuesday the Pentagon has no plans to pursue a "no-first-use" nuclear weapons policy.

His remarks at a nuclear research facility in New Mexico follow media reports saying President Barack Obama was weighing an overhaul of longstanding US nuclear policy, including by pledging to never conduct the first strike in a nuclear conflict.

"It has been the policy of the United States for a long time to extend its nuclear umbrella to friends and allies, and thereby to contribute to the deterrence of conflict and the deterrence of war," Carter said.

Though some nations including China have declared "no-first-use" policies, America and NATO allies insist that retaining the right to launch a pre-emptive nuclear strike is a vital tactical option.

The ability to strike first "has been our policy for a long time, and is part of our plans going forward," Carter said.

America's nuclear policy has been the subject of increased public discussion following a number of contentious comments by Republican presidential candidate Donald Trump, including that he wouldn't rule out the use of nuclear weapons.

The issue came up again during Monday's debate between Trump and Democratic rival Hillary Clinton.

"I would certainly not do first strike," Trump said, adding: "We have to be prepared. I can't take anything off the table."

On Tuesday, two Democratic lawmakers introduced legislation that would bar the president from launching a nuclear first strike without a declaration of war from Congress.

"Nuclear war poses the gravest risk to human survival. Unfortunately, by maintaining the option of using nuclear weapons first in a conflict, US policy increases the risk of unintended nuclear escalation," said Senator Edward Markey, who co-sponsored the bill with Congressman Ted Lieu.

"The president should not use nuclear weapons except in response to a nuclear attack."