A US Air Force Boeing 737 flew from Guam to Pyongyang with the officials via the Yellow Sea on April 7, six days before the launch went ahead, Chosun Ilbo newspaper cited a diplomatic source in Seoul as saying.

Experts speculate the aircraft carried Sydney Seiler, a National Security Council adviser to President Barack Obama, and Joseph DeTrani, director of the National Counter-Proliferation Centre, it said.

The report was one of several carried by South Korean media, although government officials and the US State Department have refused to comment.

Yonhap news agency also said the plane carried DeTrani, who organised former President Bill Clinton's visit to Pyongyang in August 2009 to secure the release of two female American journalists.

The United Nations Security Council strongly condemned the April 13 launch as breaching a ban on testing ballistic missile technology, and tightened sanctions on North Korea.

Pyongyang insists its aim was only to put a scientific satellite into orbit for peaceful purposes.

It says the launch did not breach a February agreement with the United States, which promised a suspension of uranium enrichment and a moratorium on nuclear and missile tests, in return for 240,000 tonnes of US food aid.

But the US suspended plans to start food deliveries after the launch plan was announced, saying it could not trust the North. Pyongyang in turn said it was no longer bound by the accord.

A North Korean foreign ministry statement released Tuesday appeared to refer to the secret US trip, saying that Pyongyang had informed Washington "several weeks ago" that it was exercising restraint.

The North said Tuesday it would bolster its nuclear deterrent and take "self-defence" measures unless the United States halts criticism and pressure over its rocket launches and atomic programme.

But it said a peaceful negotiated solution to the nuclear issue was still possible if the US drops its "hostile" policy.

In 2006 and 2009 the North responded with nuclear tests after the UN imposed sanctions against its rocket launches.

Satellite photos last month showed work under way at its atomic test site but gave no indication of whether or when a test would be staged.