TOKYO, Oct 12: Japanese police went on alert Thursday as North Korea threatened reprisals for wide-ranging bilateral sanctions imposed over its nuclear test.

As world powers negotiated on how to respond to North Korea, Japan late Wednesday announced its own sanctions designed to hit the impoverished regime hard including a complete ban on all imports.

Japan's police chief warned that the sanctions could trigger subversion inside the country by North Korean agents.

"It is possible that North Korea will heighten harmful activities against Japan over the duration of the sanctions," said Iwao Uruma, head of the National Police Agency, without providing further details.

Relations have long been tense between the countries due in part to North Korean agents' abductions of Japanese nationals in the 1970s and 1980s.

North Korea in 1998 fired a missile over Japan's main island, leading Tokyo to team up with the United States to develop a missile shield.

Japan's defence chief Fumio Kyuma said the two countries should consider speeding up work on the defence system, which is due to be finished by 2011.

In a possible protest, right-wing activists rammed a jeep bearing a picture of the Japanese flag into the gate of a firm accused of selling sensitive machinery on the nuclear black market.—AFP