North Korea's cyber capabilities pose serious challenges to the United States, the U.S.

cyber commander said Tuesday, emphasizing the communist nation has steadily been bolstering its capabilities and has been "quite active" in the cyber domain.

Adm. Michael S. Rogers made the assessment in a statement submitted for a Senate Armed Services Committee hearing, saying the North is one of the countries that his command is watching "most closely," along with Russia, China and Iran.

"Iran and North Korea represent lesser but still serious challenges to U.S. interests. Although both states have been more restrained in this last year in terms of cyber activity directed against us, they remain quite active and are steadily improving their capabilities," Rogers said. "Both of these nations have encouraged malicious cyber activity against the United States and their neighbors, but they currently devote the bulk of their resources and effort to working against their neighbors."

The North's cyber capabilities have been a greater focus of attention since a massive hacking attack on Sony Pictures in late 2014, which Pyongyang is believed to have carried out in retaliation for Sony's release of a comedy film ridiculing North Korean leader Kim Jong-un.

"A year ago I mentioned North Korea's brazen cyber operations to impair and intimidate Sony Pictures Entertainment. We have seen no repetition of such destructive assaults against targets in the United States," Rogers said. (Yonhap)