French President Nicolas Sarkozy branded Israeli Prime Minister Benjamin Netanyahu "a liar" in a private conversation with President Barack Obama that was accidentally broadcast to journalists during last week's G20 summit in Cannes.

"I cannot bear Netanyahu, he's a liar," Sarkozy told Obama, unaware that the microphones in their meeting room had been switched on, enabling reporters in a separate location to listen in to a simultaneous translation.

"You're fed up with him, but I have to deal with him even more often than you," Obama replied, according to the French interpreter.

The technical gaffe is likely to cause great embarrassment to all three leaders as they look to work together to intensify international pressure on Iran over its nuclear ambitions.