The Army will reportedly train thousands more troops in tunnel warfare this year on the chance President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE wants such a military option for North Korea.

U.S. officials told NPR that the Pentagon is also buying additional specialized equipment needed for tunnel operations, including radios and night vision goggles.

The U.S. Army routinely trains one or two brigades in tunnel warfare for possible closed-environment fighting in Afghanistan or elsewhere in the Middle East. With the looming threat of conflict with North Korea, however, the service is training additional troops.

Officials would not say how many additional brigades will take part in the training — which is classified information — but did say the exercises would take place throughout 2018.

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Army officials would not comment on the record to NPR on the additional training.

Pentagon officials for years have warned that North Korea has thousands of tunnels and bunkers beneath its surface, some hundreds of feet deep and possibly used to hide troops and chemical, biological and nuclear weapons.

Former Defense Secretary Donald Rumsfeld at his 2001 confirmation hearing called the North Koreans “world-class tunnelers” who “have gone underground across that country in a way that few nations have done.”

The tunnels are so far below the surface that they likely would be safe from most missiles and bombs dropped. Any military action against North Korea would include an attack on the tunnels, officials told NPR.

The Army is also buying more bridging equipment to be used in any possible military action in North Korea, as the Pentagon doesn’t want the military to bet on North Korean infrastructure when moving across rivers.

Relations between North Korea and the Trump administration have been tense in the last year due to a ramped up series of intercontinental ballistic missile launches and a hydrogen bomb test by the nation.

In addition to tunnel warfare, U.S. officials have reportedly talked about potentially conducting a targeted strike against sites in North Korea in a "bloody nose" strategy.