The French president, Nicolas Sarkozy, described the Israeli prime minister, Binyamin Netanyahu, as a "liar" in a private exchange with Barack Obama at last week's G20 summit in Cannes that was inadvertently broadcast to journalists.

"I cannot stand him. He's a liar," Sarkozy told Obama. The US president responded by saying: "You're fed up with him? I have to deal with him every day."

Neither leader apparently realised that microphones that had been attached for a press conference had already been switched on, allowing journalists waiting for a press conference to hear the conversation.

The exchange was first reported on the French website Arrêt Sur Images, and was later confirmed by a Reuters reporter who also heard the remarks.

The gaffe came before a press conference while the two presidents were in a private room.

Arrêt Sur Images reported that while the two presidents were in private discussions in a closed room before their press conference, Élysée staff handed out translation sets to waiting journalists.

A staff member reportedly explained that the headphones to go with the translation sets were not yet being handed out because this would have allowed journalists to listen in on the private conversation still going on. Half a dozen journalists immediately plugged in their own headphones and caught three minutes of the private exchange.

The conversation apparently began with Obama criticising Sarkozy for not warning the US that France would vote in favour of the Palestinians' application to join Unesco, the United Nations agency for culture and education.

One French journalist told Arrêt Sur Images that the conversation was broadcast for around three minutes before officials realised the mistake. Another told the website that the reporters agreed not to publicise the remarks because of their sensitive nature.

The exchange was mentioned on a blog by Le Monde's political correspondent Arnaud Leparmentier, who said the two presidents had discussed their "difficult" relationship with Netanyahu behind closed doors.

Spokesmen for the Élysée and Netanyahu declined to comment.