President Barack Obama said Friday he would like to further cut the US nuclear arsenal, admitting he has concerns about efforts to modernize America's most deadly weapons.

"My preference would be to bring down further our nuclear arsenal," he told reporters at the end of a Nuclear Security Summit he hosted in Washington.

Obama, who won a Nobel Peace Prize in his first year in office for his commitment to non-proliferation, was responding to a question about updates to the US arsenal.

He was asked if there was a risk that US rivals like Russia and China would feel compelled to expand their stockpiles if America improves the lethality of its own.

"I think it's a legitimate question, and I am concerned," Obama said.

"Here's the balance that we've had to strike. We have a nuclear stockpile that we have to make sure is safe and make sure is reliable."

Obama said the modernization was designed to make America's missiles more safe, and that he hoped to be able to work with powers like Russia to reduce global warhead stocks.

Nuclear terror threat 'constantly evolving': world leaders
Washington (AFP) April 1, 2016 - World leaders reaffirmed their commitment to keep nuclear weapons out of the hands of extremists on Friday but warned that the threat is "constantly evolving."

"More work remains to be done to prevent non-state actors from obtaining nuclear and other radioactive materials, which could be used for malicious purposes," the leaders said, in a joint statement at the Nuclear Security Summit in Washington.

US President Barack Obama invited dozens of world leaders to Washington for the summit, where they discussed action plans to secure nuclear fuel stockpiles and radioactive material that could be used in a nuclear device or a dirty bomb.

"We reaffirm our commitment to our shared goals of nuclear disarmament, nuclear non-proliferation and peaceful use of nuclear energy," the statement said.

"We commit to fostering a peaceful and stable international environment by reducing the threat of nuclear terrorism and strengthening nuclear security.

"Sustaining security improvements requires constant vigilance at all levels, and we pledge that our countries will continue to make nuclear security an enduring priority."

Annexed to the statement were five "action plans" designed to improve the coordination of member states with world bodies such at the International Atomic Energy Agency and the police agency Interpol.