The United States on Thursday led a wary international welcome to North Korea's declaration about its nuclear weapons, with President George W Bush calling it "an important step" as he eased sanctions against the Stalinist state.

UN Secretary General Ban Ki-moon called the North's move a "very encouraging development" but Japan urged the United States and other nations to maintain close surveillance of North Korea's disarmament moves.

The US administration made it clear it still has concerns about the North's nuclear bomb capability, but Bush eased trade sanctions and moved to take North Korea off a US terrorism blacklist after it handed over a nuclear dossier to China, six months behind schedule.

The US president stressed that global pressure must be maintained and warned the North must still come clean on its suspected arsenal and any sharing of nuclear expertise with other nations.

"Our ultimate goal remains clear: a stable and peaceful Korean Peninsula, where people are free from oppression, free from hunger and disease, and free from nuclear weapons. The journey toward that goal remains long, but today we have taken an important step in the right direction," Bush said.

North Korea will be taken off the US terrorist list after a 45-day review period.

"During this period, the United States will carefully observe North Korea's actions and act accordingly," he said, warning of further punishment for any "wrong choices."

Democratic presidential contender Barack Obama said the US should keep sanctions on Pyongyang until it proves unambiguously that its nuclear drive is over.

"We should continue to pursue the kind of direct and aggressive diplomacy with North Korea that can yield results," he said, adding that answers were needed about nuclear proliferation to Syria and other nations.

His Republican rival John McCain said North Korea's belated declaration of its nuclear activities was a "modest step forward," but agreed that sanctions should stay in place if the declaration cannot be verified.

Bush had a message for staunch ally Japan, which has insisted that Pyongyang fully account for the abductions of Japanese citizens by North Korean agents.

"We expect the North Koreans to solve this issue in a positive way for the Japanese," he said.

North Korea's declaration was part of a series of measures aimed at getting North Korea, which tested an atomic weapon in October 2006, to agree to nuclear disarmament in exchange for aid and security guarantees.

The North agreed the measures with the US, South Korea, China, Russia and Japan.

"The accounting they have given is: 'We are not now engaged in any enrichment programme or any proliferation activities and we will not in the future,'" said US national security adviser Stephen Hadley.

US Secretary of State Condoleezza Rice said North Korea must grant access to its Yongbyon plutonium reactor core and its radioactive nuclear waste.

Ban said the North's declaration and the US gestures in reply were "important steps forward."

The UN Secretary General urged all six countries involved in the Korea nuclear talks "to take this opportunity to expedite movement toward the full implementation" of a 2005 joint statement on the denuclearisation of North Korea.

Tokyo is alarmed at Washington's plans to ease sanctions against Pyongyang, analysts said, calling the US moves a diplomatic defeat for Japan.

"If the United States delists North Korea from the list of state sponsors of terrorism, it will give North Korea room to breathe," said Toshimitsu Shigemura, a North Korean expert at Waseda University.

"It will be a loss of one diplomatic card to negotiate with North Korea, which will then have no need to negotiate with Japan. Japan will be left with no leverage to discuss the abductions," he said.

But Prime Minister Yasuo Fukuda denied the claims. He said Japan and the US would continue to work together closely on the North Korean disarmament and the abductions.

"This will be really necessary to realise the denuclearisation and, at the same time, pave the way for solving the abduction issue which is a major task for our country."