North Korean ships have long been under suspicion

Pyongyang has responded angrily to plans by the international community to step up checks on North Korean ships.

North Korea, in a state-run newspaper commentary, threatened an "immediate physical retaliatory step against the US once it judges that its sovereignty is infringed upon by Washington's blockade operation".

Last week, 12 countries meeting in Madrid endorsed a US-Australian plan to crack down on what they say is North Korea's illicit trade in counterfeit money, illegal drugs and nuclear materials.

The so-called "Madrid initiative" proposes changes in international law that would enable ships and aircraft suspected of involvement in illegal activates to be stopped and searched on the high seas.

It was supported by the UK, Canada, France, Poland, Portugal, Italy, Germany, Spain, Japan and the Netherlands, as well as Australia and the United States.

Australian foreign minister, Alexander Downer, said on Monday that the initiative was not specifically aimed at North Korea.

N Korea ferry struggling against the tide

But he acknowledged it was "directly relevant" to measures being taken or under consideration aimed at stopping the shipping of nuclear weapons materials in and out of North Korea.

Rodong Sinmun said the scheme was "pre-meditated scheme to start a new war on the Korean peninsula".

Canberra has taken a keen interest in the issue following the seizure in April by Australian authorities of a North Korean boat, the Pong Su, found to be carrying 50 kg of heroin.

The US has repeatedly accused North Korea of selling missile technology around the world, and last year detained a ship carrying Scud missiles to Yemen.

Some members of the US administration have also argued that stopping illegal North Korean trade would deprive the impoverished state of a major source of income and hasten the regime's collapse.