North Korea fired six short-range projectiles into the Sea of Japan on Thursday, just hours after the United Nations passed sweeping new sanctions against Kim Jong Un’s regime as punishment for its recent nuclear test and rocket launch.

It was not immediately clear what the projectiles were — but if they were missiles, it would be a clear contravention of U.N. resolutions and a sign that North Korea is spoiling for a fight.

South Korea’s Defense Ministry said it was investigating whether the projectiles were ­artillery rockets or short-range ballistic missiles. The projectiles were fired from Wonsan, a port city on North Korea’s east coast, about 10 a.m. local time, ministry spokesman Moon Sang-gyun told reporters in Seoul.

They flew as far as 100 miles before falling into the Sea of Japan, or East Sea, according to local reports.

This appeared to be North Korea’s response to the U.N. Security Council’s unanimous adoption of harsh sanctions, some of the strongest measures ever used to pressure Pyongyang to abandon its nuclear weapons program.

The new sanctions require cargo inspections for all goods going in and out of North Korea by land, sea or air, and the measures choke off supplies of most aviation fuel for the North’s armed forces. They also ban the sale of all small arms and conventional weapons to Pyongyang and prohibit transactions that raise hard cash for North Korea through sales of its natural resources.

The tough measures — which won the support of China and Russia, North Korea’s neighbors and closest allies — come after several provocations from the regime this year.

In January, Kim ordered North Korea’s fourth nuclear test, and its first in three years. The regime claimed it had detonated a hydrogen bomb, which would be exponentially more powerful than a traditional atomic weapon, but analysts said the claim appeared vastly exaggerated.

Just a month later, Kim oversaw the launch of a long-range rocket, ostensibly part of North Korea’s space research program but widely viewed as cover for its efforts to develop a long-range ballistic missile capable of carrying a nuclear warhead to the continental United States.

As the Security Council prepared to vote on the resolution Wednesday, North Korean news media reported that Kim had visited a factory that produces ballistic missiles and related armaments.

The Thaesong Machine Factory in Nampo, southwest of Pyongyang, is one of the oldest munitions production sites in the country. Kim visited the factory with his father, Kim Jong Il, only a month before the latter’s death in 2011, according to Michael Madden, who writes the North Korea Leadership Watch blog.

At the factory, Kim Jong Un felt a “very high degree of revolutionary zeal and spirit of the workers” ahead of the Seventh Congress of the Korean Workers’ Party, due to be held in May, the official Korean Central News Agency (KCNA) ­reported.

“There will be no fortress unconquerable for us when we go with those workers,” Kim said, according to KCNA. The workers at the factory would become the “vanguard and shock brigade in the struggle for the development of the country’s economy,” Kim was reported to have said.

Much of this year’s fanfare is thought to be preparation for the May congress, which will be the first in 36 years. Analysts say that Kim, whose legitimacy as the third-generation leader of North Korea is tenuous, wants to have tangible achievements to crow about at the event.

North Korea’s neighbors hope that the new sanctions can change Kim’s calculus.

In Beijing, Chinese officials said the goal of the sanctions was to get North Korea to return to multilateral talks about nuclear disarmament.

“Sanctions themselves are not the goal,” Foreign Ministry spokesman Hong Lei said Thursday. “Dialogue and negotiations are the fundamental way to solve the North Korea issue.”

Still, even as he called for the implementation of the sanctions, Hong reiterated China’s stance on not rocking the boat in the region.

“The current situation on the Korean Peninsula is complicated and sensitive,” he said. “We hope that various parties will keep calm and restrained and will not make moves that will escalate the situation. All parties have the obligation to maintain peaceful and stable situation on the Korean Peninsula.”

South Korean President Park Geun-hye said the adoption of the “unprecedentedly tough” sanctions resolution should send a strong message about the international community’s desire for peace on the peninsula.

Meanwhile, in Tokyo, Prime Minister Shinzo Abe said Japan strongly urged North Korea to “sincerely heed the strong warnings and condemnation repeatedly expressed by the international community and to comply faithfully and fully” with the resolution “without taking further provocative actions” such as nuclear tests and ballistic missile launches.

Gu Jinglu in Beijing contributed to this report.

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