A US delegation has visited North Korea's Yongbyon nuclear complex, the first time outsiders have been allowed into the plant since UN inspectors were expelled a year ago.
The US suspects North Korea may have resumed reprocessing spent nuclear fuel rods from Yongbyon into plutonium for use in nuclear weapons, and analysts said the trip to the secretive facility may shed light on the North's nuclear capabilities.
"We did go to Yongbyon," delegation head John Lewis, who is a professor emeritus at Stanford University, told reporters in Beijing upon arrival from the North Korean capital, Pyongyang.
Lewis and others on the unofficial delegation said they did not wish to comment on what they saw or discussed with officials until they had briefed the US government.
The five-day visit by a group that also included two US Senate aides, a nuclear specialist and a former State Department envoy for North Korea, came as the United States and its allies tried to reconvene talks with North Korea to end its suspected nuclear arms programme.
Frank Jannuzi, a Senate Foreign Relations Committee aide, characterised the trip as "a good visit, a productive visit".
But it was not yet clear if, or how, it would benefit the process of talks on North Korea's nuclear weapons ambitions, analysts said.
China hosted an inconclusive round of six-party talks on the nuclear issue in August with the US, the two Koreas, Japan and Russia.
Piao Jianyi, a North Korea expert a the Chinese Academy of Social Sciences, said the Yongbyon visit probably indicated how far along North Korea's nuclear programmes had developed.
"It has a definite significance... I think, after more than a year, North Korea's nuclear development may have made some definite progress," he said.
A Western diplomat who closely follows the issue said it was interesting the authorities allowed a visit to Yongbyon, but not necessarily significant or helpful to the process.