The administration of U.S. President Barack Obama should not adopt a nuclear "no first use" doctrine at a time when North Korea increases its nuclear arsenal and threatens key U.S. allies South Korea and Japan, U.S. experts said Thursday.

Jonathan Pollack and Richard Bush, senior experts at the Brookings Institution, made the appeal in a joint article, saying the Obama administration is reportedly trying to enunciate the "no-first use" doctrine to advance Obama's nuclear-free world vision in his final months in office.

"Northeast Asia presents a clear contradiction between President Obama's non-nuclear aspirations and existing circumstances. The Republic of Korea and Japan confront the reality of a nuclear-armed North Korea," the experts said.

"Pyongyang continues to enhance its weapons inventory and the means to deliver them. It also regularly threatens Seoul and Tokyo with missile attack, potentially armed with nuclear weapons," they said, adding that both Seoul and Tokyo are strongly opposed to a U.S. "no first use" pledge.

Non-nuclear states living in the shadow of nuclear-armed adversaries have long relied on U.S. commitment to employ nuclear weapons should our allies be subject to aggression with conventional forces, while basing their own national security strategies on that pledge and foregoing indigenous development of nuclear weapons, they said.

"Though the United States possesses a wide array of non-nuclear strike options in the event of a North Korean attack directed against South Korea or Japan, any indications that the United States might be wavering from its nuclear guarantees would trigger worst-case fears that the United States, above all, would not want to stimulate," the experts said.

The U.S. should not preemptively remove the nuclear option, especially when North Korea is in overt defiance of its non-proliferation obligations and is single-mindedly intent on a building a nuclear weapons capability, they said.

"The Obama administration must therefore balance its clear desire to advance a non-nuclear legacy with Northeast Asia's inescapable realities. Enunciating a 'no first use' doctrine or a sole purpose commitment in the administration's waning months in office is a bridge too far," they said.

Obama has sought to make the initiative for building a nuclear-free world a key legacy of his presidency, launching the Nuclear Security Summit of world leaders aimed at reducing the stockpile of fissile material and keeping it out of the hands of terrorists.

The fourth and last Nuclear Security Summit was held in Washington earlier this year. (Yonhap)