TOKYO — Most North Korean ships will be barred from Japanese ports under a law that took effect yesterday, a move that will slash trade as Tokyo ratchets up pressure on the impoverished communist country over a decades-old kidnapping dispute.

The law requires all foreign ships over 100 tons to be insured against oil spills, losses and other damage to enter Japan. Few North Korean ships are believed to meet the requirement, and some are billing the measure as economic sanctions in disguise.

North Korea relies heavily on ships that travel to Japan, which carry seafood and other goods, while bringing back remittances and much needed supplies like cars and electrical appliances from pro-Pyongyang Korean residents here.

Port officials were shown boarding a North Korean vessel for a spot inspection in Maizuru port near the western city of Kyoto yesterday on public broadcaster NHK. The law, passed in June, comes amid growing public calls for economic sanctions against North Korea's government in the long-running dispute between Tokyo and Pyongyang over the North's abductions of Japanese citizens in the 1970s and 1980s.

North Korea has acknowledged kidnapping about a dozen Japanese to train its spies and allowed five to return, while claiming the rest died. But Tokyo believes some could still be alive in the North and has rejected Pyongyang's explanations as not credible.

Japan fears outright sanctions could disrupt delicate six-nation negotiations on the North's nuclear programs — but some say the shipping law could help squeeze information about the kidnappings from Pyongyang.

"It won't shoot down North Korea, but it will place a form of 'pressure' on the North, which has many uninsured ships," the Mainichi newspaper said in an editorial. Ichita Yamamoto, a ruling party member who heads a group of legislators studying possible sanctions, said the new restrictions are not enough to pinch North Korea because many of its goods are transported on Chinese ships.

"Even after (this measure) takes effect, it will not have much effect," he said, calling for more direct sanctions.

The General Association of Korean Residents in Japan said it was "problematic" that legislators had singled out North Korea when referring to the law. An official, speaking on condition of anonymity, said he hoped North Korean boats would not be discriminated against.

So far, 16 of the estimated 100 North Korean ships that call on Japanese ports each year have been certified as having acquired the necessary insurance coverage, Japan's Land and Transport Ministry said.

The ship ban, however, accompanies other steps Tokyo has taken to gain more leverage in the highly emotional abduction dispute.

Last year, it passed another law to cut off an estimated $38 million in yearly remittances sent by Korean residents in Japan. Neither law specifically mentions North Korea, but officials said they were drawn up with that nation in mind.

Japan accounts for more than 8 percent of North Korea's international trade. North Korean ships made 974 trips to Japan in 2003, the ministry said.