Japan decided Wednesday on additional economic sanctions against North Korea for six months, including a total ban on all imports from the country, with the nation remaining on high alert over speculation that Pyongyang may conduct a second nuclear test.

The measures decided on in a meeting of the Security Council of Japan also call for banning North Korean ships from entering Japanese ports and basically barring North Korean nationals from entering Japan, Chief Cabinet Secretary Yasuhisa Shiozaki said.

"I would like to ask for the public's understanding that the measures are being taken this time to protect the safety of our people and peace in Japan," Prime Minister Shinzo Abe told reporters after the council meeting. "I urge North Korea to respond soon to the concerns of Japan and others in the world."

"Its (North Korea's) increasing missile capabilities combined with a nuclear capability is a threat to the region, especially Japan," said Abe, known for his tough stance against the North.

Asked why Japan went ahead with unilateral sanctions prior to the adoption of a U.N. Security Council resolution, Abe said, "Japan has its own position as we face the greatest threat. In addition, for Japan, there is the issue of its (North Korea's) abductions" of Japanese nationals.

The ban on entry by North Koreans took effect immediately while the ban on imports and ships will become effective on Saturday, according to Shiozaki.

On the abductions, Shiozaki told a news conference, "Japan has been saying clearly that without first resolving the abduction issue there will be no normalization of diplomatic ties. The measures this time are also a clear message to North Korea, which has not responded sincerely to the abduction issue."

Japan has already imposed sanctions on North Korea over its missile test-firings, effectively prohibiting remittances from Japan to 15 entities suspected of having links with weapons of mass destruction programs and banning a port call by the cargo-passenger ferry Mangyongbong-92, the only direct passenger link between the two countries.

The total value of Japan's exports to and imports from North Korea came to 21.4 billion yen in 2005, Shiozaki said.

This accounts for a meager portion of Japan's overall two-way trade, worth 123 trillion yen.

Japan mainly imports seafood, including shellfish, sea urchins and crabs, coal and matsutake mushrooms from the country.

The number of North Korean ships' entries to Japanese ports this year up to September is about 580. It totaled about 770 last year, down from the peak year of 2002 at about 1,300, the Japan Coast Guard said.

Earlier Wednesday, Japanese government officials were trying to verify "unconfirmed information" that North Korea may conduct another nuclear test the same day, following the one it claimed to have carried out Monday.

Foreign Minister Taro Aso told a House of Councillors Budget Committee session, "We are aware of information that it may be conducted during today, but no confirmation has been obtained."

But both Abe and Shiozaki said later that the government has not confirmed any concrete signs or information that North Korea may conduct another nuclear test Wednesday, following the one it claimed to have carried out Monday.

The Japan Meteorological Agency said it detected no seismic waves coming from the direction of northern North Korea in the period from 5 a.m. to 8:30 a.m. Wednesday.

Japan has yet to confirm whether the Monday test really took place despite North Korea claiming it successfully carried out its first- ever underground nuclear test.

Meanwhile, Defense Agency Director General Fumio Kyuma told parliament Wednesday that Japan's Maritime Self-Defense Force cannot at present inspect ships going to and from North Korea as required under a draft resolution proposed Monday at the U.N. Security Council, due to limitations under domestic law.

Such inspections are possible under domestic legislation when a situation in Japan's vicinity poses a serious threat to the nation's peace and security.

If the U.N. sanctions resolution is adopted and the nuclear test verified, the government will judge whether the test qualifies as such a contingency situation.

Shiozaki also urged North Korea to promptly return to six-way nuclear talks without preconditions and to honor a 2002 bilateral pact in which Pyongyang promised to maintain a moratorium on missile launches and strengthen the peace and stability of Northeast Asia.

The stalled six-nation talks involve the two Koreas, China, the United States, Japan and Russia. Shiozaki said all the participants want North Korea to return to the table soon.