(CNN) A North Korean buildup and partial mobilization of military forces since late last week caused so much consternation at the Pentagon that top U.S. commanders reviewed the U.S. war plan for defending South Korea in case there was a sudden indication that North Korea was going to begin a war, CNN has learned.

Two U.S. officials confirmed the details of the North Korean buildup to CNN, with the U.S. now trying to assess how much of that North Korean buildup is continuing after North and South Korea reached a deal Monday in talks at the historic "truce village" inside the Demilitarized Zone.

The U.S. officials stressed that the buildup is seen as serious. The U.S. had grown increasingly worried about North Korean intentions after leader Kim Jong Un put a specific deadline on the table for South Korea to stop the propaganda broadcasts it has been blasting across the border. The broadcasts will stop as part of the fresh deal, according to the South Korean presidential office.

The buildup led to a series of urgent discussions inside the U.S. military and between the US and South Korean military about the U.S. war plan. The officials told CNN that over the last few days, U.S. commanders and military planners have reviewed the plan to ensure has been updated to include what U.S. forces are available if needed, and what actions by North Korean would trigger a U.S. military response. The U.S. has also been talking to the South Koreans to try to encourage them to deescalate the situation, the officials said.

Based on overhead satellite imagery and other intelligence gathering, the U.S. military intelligence assessment concluded that the North Koreans late last week began a limited mobilization including:

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