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TOKYO — Japan's National Security Council has discussed how to evacuate its nearly 60,000 citizens from South Korea in the event of a crisis, a government official said Friday amid rising concern over North Korea's nuclear weapons program.

North Korea denounced the U.S. on Friday for bringing "huge nuclear strategic assets" to the Korean peninsula as a U.S. aircraft carrier group headed for the region ahead of a feared sixth nuclear missile test.

Besides commercial ships and planes, Japan would want to send military aircraft and ships to assist in the evacuation if the South Korean government agreed, the official, familiar with the discussion, said. He declined to be identified because of the sensitivity of the topic.

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The NSC, in a meeting on Thursday, also discussed how to cope with a possible flood of North Korean refugees into Japan, among whom might be North Korean spies and agents, Japanese media reported.

Tension has risen since the U.S. Navy fired 59 Tomahawk missiles at a Syrian airfield last week in response to a deadly gas attack, raising concerns about U.S. President Donald Trump's plans for North Korea, which has conducted missile and nuclear tests in defiance of UN and unilateral sanctions.

Related: Trump's Options for North Korea Include Placing Nukes in South Korea

The United States has warned that its policy of "strategic patience" is over.

Japanese Chief Cabinet Secretary Yoshihide Suga told a news conference Friday that the government was always collecting and analyzing information about North Korea's moves but refrained from commenting on details.

A PAC-3 surface-to-air missile launcher unit, used to engage incoming ballistic missile threats, in Tokyo. BEHROUZ MEHRI / AFP - Getty Images

"At present, we are in close contact with the United States and South Korea and in addition to urging (the North) to refrain from provocative actions and observe relevant U.N. Security Council resolutions, we will take all necessary steps to protect our people's lives and assets," Suga said.

Japan began working on plans to respond to a potential crisis on the Korean peninsula in February, after Prime Minister Shinzo Abe met Trump at a summit in the U.S., Kyodo news agency said.

Related: North Korea Warns It Would Use Nuclear Weapons First If Threatened

A Japanese ruling party lawmaker and a government source told Reuters this week that coping with possible North Korean refugees would be among the matters for which Japan had to prepare.

But they said there was concern that any sign of actual preparations for a possible crisis would boost public anxiety.