Two senior US officials made a secret visit to North Korea in an apparent attempt to persuade it to cancel last month's long-range rocket launch, a South Korean report said Thursday.

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

Experts speculated that 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 refused to comment.

Yonhap news agency also said the plane carried DeTrani.

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

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

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

After the launch plan was announced, the United States said it would suspend the start of food deliveries.

A North Korean foreign ministry statement released Tuesday carried an apparent reference to the reported US visit.

It said Pyongyang had informed Washington "several weeks ago" that it was exercising restraint and was "taking the concerns voiced by the US into consideration".

The North in Tuesday's statement vowed to bolster its nuclear deterrent and take "self-defence" measures unless the US halts criticism and pressure.

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

In 2006 and 2009 the North reacted to UN sanctions imposed following its rocket launches by carrying out nuclear tests.

On Thursday, South Korean foreign ministry spokesman Cho Byung-Jae warned North Korea of "grave consequences" and new international sanctions if it goes ahead with a fresh nuclear test.

Defense ministry spokesman Kim Min-Seok said Pyongyang was waiting to make a "political choice" on whether to conduct a nuclear test.

"Our judgment is that North Korea can conduct a nuclear test at any time and a political choice is left," he said.

Cross-border tensions have been high following the North's virulent criticism of Seoul's leaders in recent months.

The verbal barrage is a sign of the regime's instability as new leader Kim Jong-Un bolsters his authority, the South's Unification Minister Yu Woo-Ik told Dow Jones Newswires in an interview published Thursday.

"The reason why North Koreans criticise South Korea ever more strongly, we believe, is an expression of anxiety," Yu said.

The untested Jong-Un, aged in his late 20s, took over after the death of his father Kim Jong-Il in December last year and began reshaping the government line-up.

Individuals and organisations who fear their jobs are at risk are competing to show loyalty, partly by criticising the South, said the minister, who oversees cross-border relations.

The North in recent months has mounted an unusually extreme campaign of personal abuse against the South's President Lee Myung-Bak. It has termed him a rat and "human scum", among a variety of other insults, and threatened "sacred war" to wipe out Seoul's rulers.

Yu said the North's tone would remain strong for the time being.

"I expect this kind of fidelity race will fade away as authority gets stabilised and anxiety is removed," he said, adding Seoul was still open to dialogue.