The United States government has issued a rare alert squarely blaming North Korea for a series of cyber attacks stretching back to 2009. It has warned that more are likely on the way.  

No specific victims have been named but the report refers to a group called "Hidden Cobra," previously known as "Lazarus Group," and linked to the 2014 attacks on Sony Pictures.

Security experts say it's "highly likely" Lazarus was behind the WannaCry ransomware attack that infected 300,000 computers globally last month.

North Korea routinely denies involvement in cyber attacks.

"It is ridiculous. Whenever something strange happens, it is the stereotype way of the United States and the hostile forces that kick off noisy anti-DPRK campaign," said Kim In Ryong, deputy North Korean ambassador to the United Nations, last month. 

Hacks have included espionage at South Korean finance, energy and transportation companies, and appear to be reconnaissance ahead of further attacks.