The U.S. government is concerned about cyber attacks on Japanese defense contractors including Mitsubishi Heavy Industries, which builds F-15 fighter jets, Patriot missiles and other U.S. designed weapons for Japan's military, urging such attacks be taken seriously, the U.S. embassy in Tokyo said.

We are concerned by news reports and will continue to monitor the issue, embassy spokeswoman Karen Kelley told Reuters.

Cyber attacks anywhere have potential for long-term negative impact. Cyber security must be a public-sector priority, she said.

Computers at Mitsubishi Heavy were subject to an online assault in August. Japan's biggest weapons maker failed to tell Japan's Defense Ministry at the time, however, prompting a reproach from ministry officials on Tuesday , when they told it to investigate the incident.

The ministry warned the company it may have breached contracts to supply billions of dollars of equipment by keeping quiet about the online assault.

A second Japanese military contractor, IHI Corp, which supplies engine parts for fighter planes, said its employees had been subject to a growing number of suspicious e-mails which it had informed the police about.

(Reporting by Tim Kelly; Editing by Michael Watson)