NATO Secretary General Anders Fogh Rasmussen speaks during a meeting with Russian President Dmitry Medvedev in Moscow in December 2009. Leaked U.S. diplomatic cables describe conversations between the two, apparently passed along by an informant on the NATO chief's staff. ((Sergei Karpukhin/Reuters) )

Leaked U.S. diplomatic cables appear to show that the United States has been snooping on NATO's top official using secret sources on his own staff.

Confidential cables from the U.S. mission to NATO released Friday by WikiLeaks, the site that has published many secret government memos, said American diplomats received information on the private conversations of Secretary General Anders Fogh Rasmussen from "a member of the NATO international staff."

Instead of the staffer's name, the phrase "strictly protect" was inserted in a cable dated Sept. 10, 2009. The cable dealt with Fogh Rasmussen's proposal to improve ties with Russia by establishing contacts with the Collective Security Treaty Organization, a Russia-dominated security alliance.

The cable was signed off by U.S. Ambassador Ivo Daalder.  

NATO's 28 member nations, and a number of partner countries including Russia, maintain diplomatic offices inside the alliance's sprawling compound on the outskirts of Brussels.

While their envoys regularly monitor developments within the alliance, there has been no known case in the past of a nation spying on the secretary general.

Another cable, signed by John Heffern, deputy chief of the U.S. mission, cited "a usually reliable source" as saying that a report by Fogh Rasmussen after a Dec. 17, 2010, meeting with Russian President Dmitry Medvedev and Prime Minister Vladimir Putin "conveyed additional details … that were not reflected in the secretary general's report on the visit."

"Based on information from someone present at the Moscow meetings, the source reported that while Russian President Medvedev was polite, neither he nor PM Putin had expressed any real interest in co-operating with NATO," Heffern reported. 

He contrasted that with the generally positive tone of Fogh Rasmussen's official report submitted to the 28 NATO allies about the prospects of future co-operation, particularly in Afghanistan.

No comment from NATO, U.S. 

The alliance has condemned the release of the secret diplomatic cables, which were carried by several European newspapers on Friday.

"NATO does not comment on alleged leaked documents," spokeswoman Oana Lungescu said. 

The U.S. mission to the alliance said it could not comment on the documents or their veracity.

Fogh Rasmussen, a former prime minister of Denmark, became NATO's top civilian official in 2009 as part of a team assembled by President Barack Obama to lead the war effort in Afghanistan. Others included Admiral James Stavridis, the alliance's supreme commander, and Gen. Stanley McChrystal as its U.S. commander in Afghanistan.

He has been prominent in pushing for improved relations with Russia, after ties between the alliance and Moscow hit rock bottom following the 2008 war in Georgia.