TOKYO (Reuters) - Japan’s navy and air force began a three-day military exercise with two U.S. aircraft carriers in the Sea of Japan on Thursday adding pressure on North Korea to halt an accelerating ballistic missile program.

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Japan’s Maritime Self Defence Force has sent two ships, including one of its four helicopter carriers, the Hyuga, to join the U.S carriers, the USS Ronald Reagan and USS Carl Vinson, and their eight escort ships, Japan’s military said in a release.

Japanese Air Self Defence Force F-15s are taking part in simulated combat with U.S. Navy F-18 fighters at the same time, the military said.

“It’s the first time we have exercised with two carriers. It’s a major exercise for us,” a Japanese military spokesman said.

The Sea of Japan separates Japan from the Korean peninsula.

The United States sent the warships to the region after a surge of tension on the Korean peninsula over fears the North was about to conduct a sixth nuclear test, or another test in its bid to develop an intercontinental ballistic missile capable of hitting the mainland United States.

Japanese Prime Minister Shinzo Abe has vowed to work with other countries to deter North Korea, which on Monday conducted a short-range ballistic missile test.

The missile reached an altitude of 120 km (75 miles) before falling into the Sea of Japan in international waters, but inside Japan’s exclusive economic zone where it has jurisdiction over the exploration and exploitation of maritime resources.

The launch followed two successful tests of medium-to-long-range missiles in as many weeks as North Korea conducts tests at an unprecedented pace,

North Korea can already strike anywhere in Japan with missiles, raising concern in Tokyo that it could eventually be threatened by a North Korean nuclear strike.

South Korea’s new liberal president, Moon Jae-in, who took office on May 10, has taken a more conciliatory line than Abe, pledging to engage with his reclusive neighbor in dialogue.