US President Barack Obama has revealed that his security staff wouldn't let him eat a birthday cake that was to be offered by union leaders for his 49th birthday.

Obama, who spoke at an AFL-CIO labour federation meeting in Washington earlier this week, said that "it's good to spend my birthday with some good friends", but that "the Secret Service nixed the cake".

AFL-CIO president Richard Trumka had offered the US head of state birthday wishes and said the powerful labour group wanted to provide a cake for the occasion.

"I was disappointed there wasn't a cake," Obama said.

Referring to the Secret Service detail that protects the leader around the clock, Obama said: "They're probably eating it right now".

His birthday celebration was a muted affair as First Lady Michelle Obama and youngest daughter Sasha are vacationing in Spain, while older daughter Malia is away at summer camp.

Obama himself went home to Chicago where he was scheduled to have dinner with friends.