Lee Suk-joon, South Korea’s minister of the Office for Government Policy Coordination, announces unilateral sanctions on North Korea during a news conference in Seoul. (Ahn Young-joon/AP)

North Korea has been able to make its nuclear warheads small enough to fit onto ballistic missiles, the state media claimed Wednesday in Pyongyang’s latest boast about improvements in its weapons capabilities.

North Korea has made the claim before and it is not known whether it is true, but the timing of the announcement is not auspicious.

The United States and South Korea are conducting huge military exercises that North Korea views as a pretext for an invasion, and Seoul on Tuesday unleashed a wave of unilateral sanctions against Pyongyang, including blacklisting the North’s official in charge of inter-Korean relations.

The North’s official Korean Central News Agency reported Wednesday that Kim Jong Un, the state’s leader, had visited nuclear scientists and then announced the “tremendous” achievement.

“The nuclear warheads have been standardized to be fit for ballistic missiles by miniaturizing them,” KCNA reported Kim as saying to the scientists. “This can be called [a] true nuclear deterrent. . . . Koreans can do anything if they have a will.”

Kim is trying to bolster his legitimacy ahead of a Workers’ Party congress he ordered for May, the first in 36 years. In January, he oversaw a nuclear test — Pyongyang claimed it was a hydrogen bomb — and last month he ordered the launch of a long-range rocket thought to be part of a ballistic missile program. The United Nations responded by introducing new sanctions against the North’s regime.

This is not the first time that North Korea has declared it has miniaturized a nuclear warhead, and while there is some skepticism that it has been able to make this breakthrough, there is also an assumption that it’s not unreasonable.

Gen. Curtis M. Scaparrotti, commander of U.S. Forces Korea, and Adm. William Gortney, head of the U.S. Northern Command, have said they believe North Korea has the capability to make its weapons small and light enough to fit on a missile.

The announcement came a day after South Korea’s intelligence agency said that North Korea had hacked into the smartphones of South Korean officials and stolen information from them.

The hackers had access to phone conversations and text messages, as well as the phone numbers of other senior Southern officials, the Yonhap News Agency reported. The intelligence agency, which held an emergency meeting Tuesday, did not identify whose phones were hacked or what information they contained.

In a rare statement, the intelligence agency said North Korea also tried to hack into the email accounts of railway workers in a bid to compromise the South’s train networks. But the agency said it intercepted the cyberattack.

Seoul has previously blamed the North for cyberattacks on its banks and its nuclear power operator, while the United States accused Pyongyang in the 2014 attack on Sony Pictures over the movie “The Interview,” which revolved around a plot to assassinate Kim. North Korea has denied involvement.

Also Tuesday, South Korea introduced unilateral sanctions designating 30 companies with links to the North’s nuclear and missile programs, as well as 38 North Korean nationals and two foreigners. The foreigners are Leonard Lai, president of ­Singapore-based Senat Shipping, and Lyou Jen-yi, the Taiwanese president of Royal Team Corp.

All will be banned from the South Korean financial system, and any assets they have in the South will be frozen. This is the first time that South Korea has targeted people and companies by name, a practice used by the United Nations, the United States and the European Union.

Seoul also said that it will ban from its waters ships that have been to North Korea within the past 180 days and will introduce stronger controls on imports to and exports from North Korea. The U.N. sanctions mandate inspections for all goods going in and out of North Korea by land, sea or air, and one North Korean ship has already been impounded in the Philippines.

The sanctions, which overlap with measures imposed by the United States and the United Nations, signal that South Korean President Park Geun-hye intends to inflict as much pain as possible on Pyongyang in the wake of its recent defiance.

“North Korea’s provocations — its nuclear tests and long-range missiles — can no longer be accepted, and North Korea’s misjudgments should be corrected by making them pay the heavy price for their actions,” Lee Suk-joon, minister of government policy coordination, told reporters Tuesday.

“Today’s announcement expresses the international community’s firm intention to change North Korea,” he said.

The government also renewed its advisory that South Koreans traveling abroad should not visit North Korean-owned restaurants, which are a significant source of income for the regime in Pyongyang. Seoul estimates that Pyongyang earns about $10 million a year from about 130 restaurants in a dozen countries.

Yoonjung Seo in Seoul contributed to this report.

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