Japan's National Security Council (NSC) has discussed how to evacuate its nearly 60,000 citizens from South Korea in the event of a crisis, amid rising concern over North Korea's nuclear weapons program.

North Korea denounced the United States for bringing "huge nuclear strategic assets" to the Korean peninsula, as a US aircraft carrier group headed for the region amid concerns Pyongyang may conduct a sixth nuclear test.

Besides commercial ships and planes, Japan would want to send military aircraft and ships to evacuate its citizens if the South Korean Government agreed, an official said.

He declined to be identified because of the sensitivity of the topic.

The council has previously also discussed how to cope with a possible flood of North Korean refugees into Japan, among whom there might be North Korean spies and agents, Japanese media reported.

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

The US has warned that its policy of "strategic patience" was over.

Japanese Chief Cabinet Secretary Yoshihide Suga told a news conference on Friday that the Government was always collecting and analysing information about North Korea's moves, but he did not give details.

"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 UN Security Council resolutions we will take all necessary steps to protect our people's lives and assets," Mr 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 Mr Trump at a summit in the US, Kyodo news reported.

Attendees at a February 23 NSC meeting forecast that a crisis on the Korean Peninsula could prompt large numbers of refugees to turn up in boats along the coast of the Sea of Japan, Kyodo said.

The attendees called for preparations for a humanitarian response, along with tightened security given the possibility that North Korean soldiers could enter Japan pretending to be refugees, Kyodo said, quoting unidentified government sources.

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

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

Reuters