U.S. Joint Chiefs of Staff Chairman Mike Mullen has criticized North Korea following reports that it has built a uranium enrichment facility at its nuclear site in Yongbyon.

On CNN's "State of the Union," Mullen said that such activities would violate U.N. Security Council resolutions and that the North is continuing on a path which is destabilizing for the region. Noting the North’s sinking of South Korea’s “Cheonan” warship in March, which killed 46 sailors, Mullen said that "all of this is consistent with belligerent behavior."

Also, while appearing on ABC's "This Week," Mullen said that he's been worried about North Korea's potential nuclear capability for a long time and that the latest report gives that potential real visible life.

The U.S., which believes the North's building of the facility is a new bargaining chip, has begun talks with other nations on a joint response.

A U.S. delegation led by Special Representative for North Korea Policy Stephen Bosworth is in the middle of a trip to South Korea, China and Japan to discuss the issue.

A top official of the Obama administration, meanwhile, says that North Korea has used missile launches and nuclear tests as pressure tactics in the past, but that such moves failed to be effective due to unified responses by the international community.