On Tuesday, as the new insurance liability law became effective, Japanese Transportation Ministry officials boarded foreign vessels in ports across the country for what they called random inspections.

They were checking ships of more than 100 tons to see if they had insurance against oil spills and losses, which is now required before a ship can enter Japan.

The inspectors made a show of scrutinizing vessels from numerous countries. However, the new law is seen as targeting North Korea, since less than three percent of the ships from that country making port calls in Japan have the required insurance. Russian ships will also be affected, but about three-quarters of all foreign ships that enter Japan are believed to be in compliance.

The legislation was proposed after several uninsured North Korean ships ran aground in Japan, leaving local communities to foot the cost of the cleanup.

Transport Minister Kazuo Kitagawa says his ministry will watch carefully to see if the law has any impact on local economies, but there is no indication the government will back down. Mr. Kitagawa says the new law will be strictly enforced.

The rusty fleet of ships crossing the Sea of Japan provides a significant source of Pyongyang's export revenue. The new law is expected to significantly reduce imports of North Korean seafood and other marine products into Japan.

The ships are also North Korea's lifeline for much-needed supplies: they leave Japan with remittances from ethnic Koreans, and supplies ranging from second-hand bicycles to delicacies for Communist Party leaders.

Japanese Prime Minister Junichiro Koizumi has been reluctant to impose full-scale sanctions against North Korea, despite public sentiment to do so. Japan, along with the United States, South Korea, China and Russia are still hoping to draw North Korea back into talks about its nuclear weapons programs.

While Japan's government denies the new shipping law specifically targets North Korea, however, many politicians openly admit that is the intent.

Public sentiment here against North Korea is strong, because of the highly emotional case of Japanese civilians abducted by North Korean agents during the 1970's and 1980's.

Pyongyang has admitted it kidnapped 12 Japanese to train its spies. Five were finally allowed to return to Japan in 2002, and Pyongyang says the other seven died. It says it has no knowledge of additional citizens Japan says are missing.

The abductees' families and their supporters have rejected North Korea's explanation, pointing out that officials there have repeatedly changed their stories about the fate of the missing.

Some who want to punish North Korea say the de facto restrictions will not be effective, predicting North Korea will rely more heavily on trade with Japan conducted via Chinese ships.