U.S. President Barack Obama on Tuesday urged other states to continue combating nuclear terrorism, saying the threat is "one of the gravest dangers of our time."

The president was speaking at the first plenary session of the Nuclear Security Summit in Seoul, which has drawn representatives from 53 nations and four international organizations to discuss countermeasures to nuclear terrorism.

First held in Washington in 2010, the biennial summit is a product of Obama's vision of a world free of nuclear weapons.

"There were those who questioned whether our nations could summon the will to confront one of the gravest dangers of our time," Obama told the other leaders at the meeting, according to a transcript released by the White House. "There was some skepticism that we would be able to sustain an effort around this topic. But that's exactly what we've done."

Several countries have taken active steps to prevent terrorists from getting hold of loose nuclear material, but the president warned that the work is not yet complete.

"What's also undeniable is that the threat remains. There are still too many bad actors in search of these dangerous materials, and these dangerous materials are still vulnerable in too many places," Obama said.

The successive summits are becoming "part of a larger global architecture" aimed at reducing the dangers of nuclear terrorism, he added.

"I think we all understand that no one nation can do this alone. This is one of those challenges in our interconnected world that can only be met when we work as an international community." (Yonhap News)