"We will unequivocally defend -- we are unequivocally committed -- to that alliance with South Korea as well as our other allies in that region of the world," Hagel told reporters.
"We will be prepared -- we have to be prepared -- to deal with any eventuality," Hagel said after the United States flew two nuclear-capable B-2 stealth bombers on a mission over South Korea.
"We must make clear that these provocations by the North are taken by us very seriously and we'll respond to that," Hagel said, according to the AFP news agency.
Tensions have risen sharply with North Korea, which in recent months launched a long-range rocket, tested its third nuclear bomb and threatened to attack US bases in the Pacific as well as the American mainland.
"I think their very provocative actions and belligerent tone has ratcheted up the danger," Hagel said.
The B-2 overflights were considered a major step as the United States rarely publicly announces such tests, which anger Pyongyang.
Hagel denied he United States was acting provocatively, instead maintaining that the "exercises are mostly to assure our allies that they can count on us to be prepared and to help them deter conflict."
American policymakers have expressed concern that South Korea or Japan might seek nuclear weapons of their own in response to the threat posed by North Korea.