In September 2008, thousands of North Koreans turned coloured cards to form the symbol for the atom as gymnasts performed on the field below during a "mass games" performance at a stadium in Pyongyang. (David Guttenfelder/Associated Press)

North Korea has hardened its stance on disarmament, saying it has "weaponized" plutonium into warheads, but hopes for better ties with U.S. president-elect Barack Obama, an American researcher who visited the North said Saturday.

Officials say the weapons cannot be inspected and Pyongyang might keep them even if it normalizes relations with Washington, said Selig Harrison, director of the Washington-based Center for International Policy's Asia program.

Harrison, who has visited the North 11 times since 1972, said he met this week with the North's nuclear envoy, Ri Gun, and other officials.

The officials said "North Korea is now a nuclear weapons state and will not commit itself now on when it will give it up as a result of denuclearization negotiations," Harrison told reporters in Beijing.

"We are not in a position to say when we will abandon nuclear weapons," he quoted Ri as saying.

Harrison said the North's "much, much harder line" might be due to the rise of military hardliners after leader Kim Jong Il suffered a stroke.

North Korea has denied reports that Kim had been seriously ill, but Harrison said that based on information from his own sources in Pyongyang he believed Kim suffered a stroke.

"He has recovered and he is now making what is described to me as 'key decisions' but is not dealing on a day-to-day basis with detailed issues as he had done before," Harrison said.

Foreign Minister Pak Ui Chun and others said the North wants better relations with Obama's government, according to Harrison. He said they want Obama to make sure the North receives promised energy aid and to provide help to revive North Korean agriculture.

He said the North wants its symphony orchestra invited to perform in the United States following a Pyongyang concert last March by the New York Philharmonic.

Pyongyang has made normalizing ties with Washington a priority but the United States says it must disarm first. The two governments have never had formal relations.

The latest round of six-nation talks aimed at stripping the North of nuclear capability broke down in December.

The participants are the two Koreas, the United States, China, Russia and Japan. The North agreed earlier to disarm in exchange for aid, but that deal became deadlocked over how to verify North Korea's past nuclear activities.