SEOUL - COMMUNIST North Korea threatened on Monday to take 'ultra-hardline' steps if Japan strengthens sanctions against it.

'The DPRK (North Korea) will respond in a ultra-hardline manner to Japan's pressure and strengthened sanctions', the ruling party newspaper Rodong Sinmun said in a commentary without giving details.

Japan's sanctions - imposed after the North tested an atom bomb in October 2006 for the first time - include bans on all imports from the impoverished state and all port calls by North Korean-registered ships.

Japanese Prime Minister Taro Aso has reportedly suggested that Tokyo may consider more sanctions to press Pyongyang to account for Japanese civilians kidnapped in the 1970s and 1980s to train the North's spies.

North Korea agreed in June to start a new probe into the abductions but said in September, before the conservative Aso came to power, that it would delay the investigation.

Rodong described as 'nonsense' any attempt by Japan to impose more pressure or additional sanctions.

'The DPRK has stayed alive under Japanese sanctions. It has built up solid grounds for a self-reliant national economy despite persistent sanctions,' said the paper, quoted by the official Korean Central News Agency.

'Pressure or sanctions will not pose any threat to us'. North Korea in 2002 admitted abducting some Japanese and has allowed five victims to go home, but Japan contends that more are being kept under wraps.

It has refused to provide aid to North Korea under a six-nation disarmament deal until there is progress on the abductions. -- AFP