U.S. renunciation of the preemptive nuclear strike option would be "deeply destabilizing" at a time when Russia is flexing its military muscles, China is building up its nuclear forces and North Korea is bent on developing nuclear missiles, a top security expert said Monday.

Andrew Shearer, senior Asia Pacific security adviser at the Center for Strategic and International Studies, raised the concern as the administration of President Barack Obama is reportedly considering adopting a nuclear "no first use" doctrine to bolster his legacy as champion of a world without nuclear weapons.

"Proponents of these ideas claim they would reduce tensions and the risk of escalation or inadvertent nuclear conflict.

Unfortunately, the opposite is true: Despite their superficial appeal, these measures would be deeply destabilizing," Shearer said in an article on the "War on the Rocks" website.

A no-first use policy is unlikely to persuade U.S. nuclear rivals to change their plans, could "needlessly tie a president's hands," and creates a perverse incentive for adversaries to strike first, said Shearer, who served as national security adviser to Australian prime ministers John Howard and Tony Abbott.

"Russia is modernizing its strategic forces and openly threatens to use nuclear weapons in eastern Europe. It is easy to dismiss this as bluster, but President Putin has already used force to change Europe's borders ? the first time this has occurred since 1945. Western military planners worry that conventional NATO forces in Eastern Europe may be insufficient to deter a Russian move on the Baltics, even though they are being augmented," Shearer said.

"In Asia, North Korea's unpredictable dictator Kim Jong-un already has nuclear weapons, and is developing land and submarine-delivered missiles to launch warheads at targets in Asia, the Pacific, and as far as the American mainland. Last week, a missile landed within 200 miles of Japan," he said.

In addition, China is also modernizing, diversifying, and building up its nuclear forces, while deploying new conventional weapons to constrain U.S. forces' ability to project power and defend allies in the Western Pacific, the expert said.

The no-first use policy would also lead to allies questioning the U.S. security commitment to them at a time when Republican presidential nominee Donald Trump is "bashing longstanding allies and telling Japan and the Republic of Korea to get their own nuclear weapons," he said.

"Allies don't vote, but they shouldn't be ignored. By providing extended deterrence, U.S. nuclear weapons not only defend the United States but also reassure allies and help to convince them not to obtain their own. Rising tensions and instability in Europe, Asia, and the Middle East mean that American allies already face significant threats," Shearer said.

"Giving them cause to doubt the reliability of the U.S. nuclear deterrent will only feed their insecurity and make them less dependable partners. It will also stoke strategic competition in Asia and the Middle East, spur proliferation, and increase the risk that nuclear weapons are used some day ? the precise opposite of what the White House intends. Japan has already expressed concern," he said.

Rather than changing the nuclear doctrine, Obama should focus on boosting conventional deterrence, reassuring American allies and tightening the screws on North Korea, Shearer said.

"U.S. presidents occasionally get the nuclear wobbles, generally for good motives. Ronald Reagan detested nuclear weapons and famously almost bargained them away with Mikhail Gorbachev. But he didn't. Instead his legacy was the peaceful defeat and collapse of Soviet expansionism and the end of the Cold War," he 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)