The United States is known to have deployed stealth bombers in a base in Guam amid North Korea’s threats of a rocket launch.

Military sources in Washington said the U.S. Air Force deployed four of the radar-evading B-2 stealth bombers at Guam’s Andersen Air Force Base to replace a rotational B-52 bomber unit.

Experts said the B-2 bombers are likely to join an annual drill between the U.S. and South Korea slated for March 9.

The U.S. Air Force operates 20 B-2 bombers capable of penetrating sophisticated air-defense shields.