While Hagel said the step was part of normal exercise, he did acknowledge that North Korea's actions in recent weeks have ratcheted up danger in the region, "and we have to understand that reality." He said the mission was designed to send a message of support for allies.
"The North Koreans have to understand that what they're doing is very dangerous," Hagel said. "I don't think we're doing anything extraordinary or provocative or out of the ... orbit of what nations do to protect their own interests." The U.S., he added, must make it clear to South Korea, Japan and other allies in the region that "these provocations by the North are taken by us very seriously and we'll respond to that."
The two B-2 bombers from a U.S. air base in Missouri dropped dummy bombs on a South Korea island range before returning home. According to the Pentagon, this was the first time that dummy munitions were dropped.
In recent weeks, Pyongyang has threatened to carry out nuclear strikes on Washington and Seoul to highlight displeasure over the drills and U.N. sanctions over its nuclear test last month.
Hagel said there are a lot of "unknowns" with North Korea and its new president Kim Jong Un.
"But, we have to take seriously every provocative, bellicose word and action that this new, young leader has taken so far since he's come to power," Hagel said.
So far, officials have not seen North Korea take any actual threatening military steps in response to the bombers.
The Associated Press contributed to this report.