WASHINGTON (AFP) – The chief of US intelligence has warned spy agencies against "blabbing secrets" to the media, saying employees should be "seen not heard."

In a blunt memo, James Clapper, the new director of national intelligence, scolded staff members about leaks that appeared in recent news reports, saying it was a "serious matter."

"I am concerned that recent leaks regarding our work have received prominent attention in the media," he said in the memo obtained by AFP.

He did not say to which reports he was referring, but US newspapers have recently quoted unnamed officials about proposed drone CIA operations in Yemen and Afghan officials allegedly on the spy agency's payroll.

Top US officials were also stunned earlier in July by the release of tens of thousands of secret files on the war in Afghanistan, posted on the WikiLeaks website.

There are "established procedures for authorized officers to interact with the media," Clapper wrote.

But for other personnel, passing on classified information without approval "is both a serious matter and a diversion from the critical tasks we face.

"In other words, blabbing secrets to the media is not 'in' as far as I'm concerned."

Clapper recalled that when President Barack Obama nominated him to the intelligence director's position, "I said that people in the intelligence business should be like my grandchildren -- seen but not heard."

The Obama administration has adopted a tough line against leaks, filing charges against those suspected of disclosing classified information.

But news reporting suggests government officials continue to reveal secret details to journalists in an attempt to shape policy and undermine rival agencies.

The memo was the latest sign "that the administration remains vexed by leaks," said Steven Aftergood of the Federation of American Scientists, who writes a blog on government secrecy.

"But the memo itself is just a reminder, and doesn't seem to represent a new policy. The pending prosecutions send a more ominous signal," he told AFP in an email.

Last week, the Justice Department unveiled an indictment against a State Department contractor, Stephen Jin-Woo Kim, for allegedly passing on defense information. Kim has pleaded not guilty.

The case reportedly involves a 2009 intelligence assessment given to Fox News, saying that North Korea was likely to respond to UN sanctions by launching another nuclear test.

US Defense Secretary Robert Gates in July also gave a stern warning to the Pentagon work force over the media, saying those who violated the law would be prosecuted.