South Korea accuses Pyongyang of planning nuclear weapons test

South Korea has accused its northern neighbour of planning another nuclear weapons test, even as it prepares for the launch of a long-range rocket that has itself sparked condemnation.

Last month, North Korea announced that it was preparing to launch a rocket that would carry a satellite into space. The launch will coincide with celebrations beginning on Thursday of the 100th birthday of the country's founder, Kim Il-sung.

South Korea, Japan and the US have condemned the launch, accusing Pyongyang of using it as cover for the testing of long-range missiles that would breach a UN resolution. Now, the authorities in Seoul say they believe the North is preparing for what would be its third nuclear test. A report from the country's intelligence agencies accused the North of "covertly preparing [for the launch] which would be another grave provocation".

"North Korea is digging up a new underground tunnel at the Punggye-ri nuclear test site, in addition to its existing two underground tunnels, and it has been confirmed that the excavation works are in the final stages," the report said, according to a copy seen by the Associated Press.

Satellite images apparently show digging at Kilju county in the north-east, the site of the two previous nuclear tests in 2006 and 2009.

The report said soil is needed to fill up underground tunnels before a nuclear test and indicated a "high possibility" North Korea was preparing for such a move.

North Korea has yet to respond to the latest claims and there has been no independent evidence to support the allegation from South Korea. Last month, the North said the launch of a rocket due to take place between 12 and 16 April to mark the centenary of Kim Il-sung's birth would help the "building of a thriving nation". It said the satellite would be used to collect information on crops, forests and weather.

Three years ago, North Korea pulled out of six-party nuclear-disarmament talks. However, in February, in a move that encouraged regional observers, it agreed to halt nuclear tests, uranium enrichment and long-range-missile launches in return for food aid. That arrangement will be scuppered if – as the US, South Korea and Japan allege – the imminent launch is of a missile rather than simply a satellite.

Two previous launches of the long-range missile have failed. The most recent attempted launch was in 2009.

President Barack Obama last month called on North Korea to end its nuclear ambitions or face further international isolation, but the US administration appears to have no clear idea how to respond to the prospect of a North Korean rocket launch – and possible third nuclear test. President Obama is focused on domestic politics and his forthcoming campaign for re-election.

"It does not appear to me that North Korea has been occupying a whole lot of time and attention," said Scott Snyder, Korea analyst at the Council on Foreign Relations. "It looks like they're going to wait and watch."

In Seoul, Choi Jin-wook, North Korea expert at the Korea Institute for National Unification, questioned whether North Korea would go through with a third nuclear test in view of the moratorium negotiated by the US and North Korea in February. "We cannot expect a catastrophic situation from a rocket launch," Mr Choi said, "but if they test an underground nuclear device, that is much more serious."

He was, however, wary of the reports that the North was planning a nuclear test, and predicted that after the furore surrounding the launch had died down, the US and North Korea would return to negotiating a moratorium in exchange for food aid.