Story highlights The U.S. military had been observing the submarine operate off North Korea's east coast when the vessel stopped

Heightened tensions on the Korean peninsula following a fourth North Korean nuclear test

U.S. and South Korea holding largest ever military drills

Washington (CNN) The North Korean regime lost contact with one of its submarines earlier this week, three U.S. officials familiar with the latest information told CNN.

The U.S. military had been observing the submarine operate off North Korea's east coast when the vessel stopped, and U.S. spy satellites, aircraft and ships have been secretly watching for days as the North Korean navy searched for the missing sub.

The U.S. is unsure if the missing vessel is adrift under the sea or whether it has sunk, the officials said, but believes it suffered some type of failure during an exercise.

Tensions have heightened on the Korean peninsula following a fourth North Korean nuclear test and joint U.S.-South Korean military drills.

On Thursday, the South Korean military said North Korea fired two short-range ballistic missiles . They were fired from North Hwanghae province, south of Pyongyang, toward the sea east of the Korean Peninsula, the South Korean Joint Chiefs of Staff said. The South tracked the projectiles and is monitoring the situation, it said.

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