North Koreans are consistently trying to break into US servers for classified information, US Secretary of State Mike Pompeo said Monday, adding North Korea and China pose greater threats than Russia to the US.

"It's an ongoing battle, an ongoing struggle to keep our systems safe," the top US diplomat said in an interview with US radio network Breitbart News.

Pompeo said the United States faces similar threats from Russian and Chinese actors.

"I can't say much other than it's been a consistent effort of the Russians to try and get into American servers, not only those of government agencies but of businesses. We see this even more strongly from the Chinese Communist Party, from the North Koreans as well," he said, according to a transcript of his remarks released by the State Department.

"I'm very confident the United States Government will keep our classified information out of the hands of these bad actors," added Pompeo.

The US has this year alone issued several warnings against activities by North Korean hackers, with most of their hacking attempts targeting banks or international financial networks to steal money.

John Demers, assistant attorney general for national security, earlier noted the North's pursuit of financial gains through hacking made it "unique" from other countries such as China or Russia.

"That's not really the behavior we see from China or Russia, or even Iran, despite the Iran sanctions. North Korea is kind of unique in that way," Demers has said.

Pyongyang is said to have more than 6,000 trained hackers, most of whom are based in other countries, including China and Russia. (Yonhap)