Obama pledges nuclear cuts, warns N Korea and Iran

US President Barack Obama said today the United States will further cut nuclear stockpiles as he seeks global momentum for measures to keep atomic weapons out of extremist hands at a summit overshadowed by North Korea.

In a speech hours before the opening of a nuclear security summit in South Korea, Obama said major progress had been made over the past two years to eliminate or safeguard material that could be used to make thousands of bombs.

"But we're under no illusions. We know that nuclear material -- enough for many weapons -- is still being stored without adequate protection," he said.

"We know that terrorists and criminal gangs are still trying to get their hands on it, as well as the radioactive material for a dirty bomb. The danger of nuclear terrorism remains one of the greatest threats to global security."

Obama urged the leaders or top officials from 53 nations gathered in Seoul for the summit to "keep at it", and pledged further actions from the United States including efforts with Russia to jointly cut their stockpiles.

He said that, with more than 1,500 deployed nuclear weapons and 5,000 warheads, the United States had "more nuclear weapons than we need".

"I firmly believe that we can ensure the security of the United States and our allies, maintain a strong deterrent against any threat, and still pursue further reductions in our nuclear arsenal," he said.

The atomic programmes of North Korea and Iran were also set to be the focus of intense discussion on the sidelines of the summit, although they were not officially on the agenda of the two-day event.