The top U.S. military officer said on Monday the United States was vulnerable to cyber attacks, and called for more aggressive action to bolster America's online defenses.

The comments by General Martin Dempsey, chairman of the Joint Chiefs of Staff, were the latest by U.S. military officials flagging cyber security as an area of growing focus and investment even as the Pentagon braces for declining budgets.

We lose enormous intellectual property rights. We're under constant attack every day. And it's going to take a whole government approach, Dempsey told a forum in London, in what was billed as his first full-scale address since taking over the Pentagon's top uniformed job in September.

Recent attacks on U.S. corporations such as Google Inc, the Nasdaq stock exchange, Lockheed Martin Corp, and RSA, the security division of EMC Corp, have given U.S. government and military officials a renewed sense of urgency about addressing threats to U.S. computer networks.

An arm of the U.S. intelligence community released a report earlier in November identifying China and Russia as the most active and persistent nations that are using cyber espionage to steal U.S. trade and technology secrets.

But data theft is only one area of concern. U.S. officials stepped up warnings about possible destructive cyber attacks after the computer virus Stuxnet emerged in 2010.

Stuxnet is believed to have crippled centrifuges that Iran uses to enrich uranium for what the United States and some European nations have charged is a covert nuclear weapons program.

We are not immune to coercion in cyber. And we have to get after it, Dempsey said. We're working on it .... but in my judgment we need to work harder.

(Editing by Peter Graff)