North Korea has warned a western-backed report blaming it for the sinking of a South Korean navy ship in March has left the peninsula in a "phase of war".

On Thursday a group of international experts said there was "no other plausible explanation" for the sinking of the 1,200-tonne Cheonan on March 26th that left 46 South Korean sailors dead other than a torpedo fired by a North Korean submarine.

Pyongyang responded by threatening "all-out war" if the south responded with any military action, but Seoul has said it will pursue only a tightening of sanctions.

In response, the north's Korean Central news agency, quoting a top-ranking governmental committee, said: "From this time on, we will regard the situation as a phase of war and will be responding resolutely to all problems in north-south relations.

"If the south puppet group comes out with 'response' and 'retaliation', we will respond strongly with ruthless punishment including the total shutdown of north-south ties, abrogation of the north-south agreement on non-aggression and abolition of all north-south cooperation projects."

South Korean president Lee Myung-bak is facing growing domestic pressure to respond decisively to the north's actions, but he has pledged a "very prudent response".

In the UK, foreign secretary William Hague condemned Pyongyang for the "callous act" of sinking the Cheonan.

"UK experts have been working alongside South Korean and other international partners on the investigation into the sinking of the Cheonan," he said.

"They have been impressed with the objectivity and rigour of the investigative work. The UK experts are in no doubt as to the veracity of the investigation's findings.

"The Democratic People's Republic of Korea's actions will deepen the international community's mistrust. The attack demonstrates a total indifference to human life and a blatant disregard of international obligations."

Analysts have warned of a growing rift between the key respective allies of Pyongyang and Seoul, with China refusing to endorse the findings of Thursday's report and the US condemning the north for carrying out the attack.

US secretary of state Hillary Clinton is due to travel to Asia this week and is scheduled to visit Seoul on May 26th.
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