The U.S. military chief on Thursday denounced North Korea's detention of an American college student as a "reflection of the absolutely irresponsible leadership" in the communist nation.

Earlier this week, the North's highest court sentenced the 21-year-old University of Virginia student, Otto Warmbier, to 15 years of hard labor for allegedly stealing a political propaganda sign from a hotel. He has been detained since early January.

"I think it's just a reflection of the absolutely irresponsible leadership in North Korea and it exposes the regime to those who may not have appreciated what the regime is," Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford said during a Senate Armed Forces Committee hearing.

"It exposes the regime to those who may not have appreciated what the regime is. That behavior was certainly not a surprise to me in terms of North Korean regime behavior, and I think that probably many other people ... have now seen what North Korea is all about," he said.

Dunford also said that the student is believed to be part of a religious group.

The White House called for the release of the student, saying the allegations on which he was arrested and imprisoned "would not give rise to arrest or imprisonment in the United States or in just about any other country in the world." It also accused the North of using detained Americans as "pawns to pursue a political agenda."

American visitors have often been detained in North Korea on charges of anti-state and other unspecified crimes. Widespread views are that the communist nation has used the detentions as bargaining chips in its negotiations with Washington.

Kenneth Bae, a Korean-American missionary, served two years of detention in the North before being released in November 2014 when U.S. Director of National Intelligence James Clapper made a secret trip to Pyongyang to win his release and that of another U.S. detainee.

State Department spokesman John Kirby said the U.S. is following the latest detention closely.

"We've been able to establish contact with him, which we're grateful for, and we're tracking this case very, very closely.

Sadly, we've seen this sort of unjust procedure in the past with other citizens of other countries to include U.S. citizens," he said in an interview with MSNBC.

"We're focused very keenly on this and we're going to do everything we can to help."

In a written statement submitted to the committee, Dunford said that the U.S. maintains a competitive military advantage against the relatively low-technology North Korean military. Still, however, a conflict would result in large number of casualties, he said.

"In the event of a conflict on the peninsula, North Korea may be able to seize the initiative and rapidly escalate hostilities utilizing special operations forces, mass, and long-range fires.

Risk of large numbers of civilian and military casualties remains high," he said.

In addition to its nuclear and missile programs, Dunford said the North is also expected to continue investing in more capable cyber-tools to develop asymmetric options that can be effective against more sophisticated networks.

"In summary, North Korea's ballistic missile and nuclear developments, willingness to conduct malicious cyber activities, and potential to seize the initiative in a conflict on the peninsula pose risks to the security of the United States and our allies," he said. (Yonhap)