Carla Bruni-Sarkozy is "excited" but "nervous" about performing in person for the first time since becoming France's first lady at a celebrity-studded concert in New York to mark Nelson Mandela's 91st birthday.

Dave Stewart, the British singer-songwriter who will perform two duets with Miss Bruni on Saturday night and is a driving force behind the Mandela Day celebration, offered the insights on his friend after he spoke to her in Paris.

"Carla's excited and a bit nervous," said Stewart, who rose to fame with Annie Lennox as The Eurythmics.