France believed the United States attempted to hack into its president's communications network, a leaked US intelligence document published on Friday suggests.

US agents denied having anything to do with a May 2012 cyber attack on the Elysee Palace, the official residence of French presidents, and appeared to hint at the possible involvement of Mossad, Israel's intelligence agency, a classified internal note from the US National Security Agency suggests.

Extracts from the document, the latest to emerge from the NSA via former contractor Edward Snowden, were published by Le Monde newspaper alongside an article jointly authored by Glenn Greenwald, the US journalist who has been principally responsible for a still-unravelling scandal over large-scale US snooping on individuals and political leaders all over the world.

The document is a briefing note prepared in April this year for NSA officials who were due to meet two senior figures from France's external intelligence agency, the DGSE. The French agents had travelled to Washington to demand explanations over their discovery in May 2012 of attempts to compromise the Elysee's communications systems.

The note says that the branch of the NSA which handles cyber attacks, Tailored Access Operations (TAO), had confirmed that it had not carried out the attack and says that most of its closest allies (Australia, Britain, Canada and New Zealand) had also denied involvement.

It goes on to note: "TAO intentionally did not ask either Mossad or (Israel's cyber intelligence unit) ISNU whether they were involved as France is not an approved target for joint discussions."

Le Monde interpreted this sentence as being an ironic reference to a strong likelihood that Mossad had been behind the attack.

The cyber attacks on the Elysee took place in the final weeks of Nicolas Sarkozy's term, between the two rounds of the presidential election which he ended up losing to Francois Hollande.

The attacks had been previously reported by French media, who have described them as an attempt to insert monitoring devices into the system but it remains unclear whether the presidential networks were compromised for any time.

There was no immediate response from the Elysee on Friday when asked for comment by AFP.

Sarkozy enjoyed warmer relations with the United States than any French president of recent times, to the extent that the media sometimes referred to him as "Sarko the American."

The revelations about the Elysee attacks followed damaging revelations that the US had tapped the mobile phone of German Chancellor Angela Merkel and spied on other allies.

"Spying between friends, that's just not done," Merkel said Thursday at the -start of a summit of European Union leaders which has been overshadowed by the issue.

On a lighter note, the leaked document published by Le Monde on Friday underlines that NSA officials were anxious not to cause any further offence to their angry French counterparts.

Along with the technical details, the briefing note contains a phonetic guide to the pronounciation of the names of the French visitors.

They included DGSE technical director Bernard Barbier, who was to be addressed as bear-NAR bur-BYAY, and Patrick Pailloux, or pah-TREEK pie-YOO