The Chinese are the world's "most active and persistent perpetrators" of economic espionage, a US intelligence agency said on Thursday in an unusually blunt report on a sensitive topic.

The report on foreign cyber spying efforts submitted to the US Congress also pointed the finger at Russia's intelligence services, saying they were snooping on US companies for economic information and technology.

The report, "Foreign Spies Stealing US Economic Secrets in Cyberspace," was compiled by the office of the National Counterintelligence Executive, part of the office of the Director of National Intelligence.

It included contributions from a number of US government agencies including the Central Intelligence Agency, the National Security Agency and the Federal Bureau of Investigation, as well as the private sector.

The National Counterintelligence Executive is responsible for defending against foreign intelligence threats to the United States.

Those threats increasingly involve the theft of US intellectual property and trade secrets, according to the report, which said cyber espionage has become "a larger threat than more traditional spying methods."

"Foreign economic collection and industrial espionage against the United States represent significant and growing threats to the nation's prosperity and security," the report said.

Cyberspace "amplifies" these threats by making it possible to "quickly steal and transfer massive quantities of data while remaining anonymous and hard to detect," it said.

Cyber spies are interested in information and communications technology, business information, military technologies, particularly marine systems and drones, and medical and pharmaceutical secrets, the report said.

While emphasizing that it was difficult to prove state sponsorship in cyberspace, the report said "Chinese actors are the world's most active and persistent perpetrators of economic espionage.

"US private sector firms and cybersecurity specialists have reported an onslaught of computer network intrusions that have originated in China, but the (US intelligence community) cannot confirm who was responsible," it said.

Between 2009 and 2011, "computer networks of a broad array of US government agencies, private companies, universities, and other institutions -- all holding large volumes of sensitive economic information -- were targeted by cyber espionage," the report said. "Much of this activity appears to have originated in China."

The counterintelligence agency also said "Russia’s intelligence services are conducting a range of activities to collect economic information and technology from US targets."

"Moscow's highly capable intelligence services are using HUMINT (human intelligence), cyber, and other operations to collect economic information and technology to support Russia's economic development and security," it said.

The United States is generally reluctant to level such direct accusations at China and Russia and Robert Bryant, head of the National Counterintelligence Office, was asked by reporters about the unusual candor of the report.

"What I'm looking for is solutions to what I see is a very serious problem facing the economic viability of the United States of America to go forward," Bryant said at an event held at the National Press Club to release the report

"The nations of China and Russia, through their intelligence services and through their corporations, are attacking our research and development," he said. "We want to basically point out what the issue is."

Mike Rogers, a Republican from Michigan who chairs the House Permanent Select Committee on Intelligence, said the report implicating China in industrial espionage "confirms what I have heard time and time again."

"This once again underscores the need for America's allies across Asia and Europe to join forces to pressure Beijing to end this illegal behavior," Rogers said in a statement.

The report said US intelligence agencies expect China and Russia to "remain aggressive and capable collectors of sensitive US economic information and technologies, particularly in cyberspace.

"Both will almost certainly continue to deploy significant resources and a wide array of tactics to acquire this information from US sources, motivated by the desire to achieve economic, strategic, and military parity with the United States," it said.

China has repeatedly denied state involvement in cyber espionage against Western governments and companies, including well-publicized attacks on Internet giant Google that sparked a row between Washington and Beijing.