"It's a matter of age," Gates, 64, told reporters at the Pentagon. "I thought U2 was an airplane."

Gates described Bono as a "very serious guy" and said he had been impressed with the rock singer's knowledge of Africa during their meeting on Tuesday.

Bono has met frequently with world leaders to push for spending on foreign aid and debt relief. But this was his first meeting with Gates, a former director of the CIA who replaced Donald Rumsfeld at the Pentagon in December 2006.

Gates said Bono had raised concerns he had heard about the Pentagon's creation of a new U.S. military command for Africa, known as Africom.

Aid agencies have said the command could put a military face on U.S. foreign policy toward Africa.

"We gave him some information on Africom and he was not hostile to it," Gates said. "I would say that he was open-minded about it and just recommended that we take care in how we proceed."

Gates said he did not have a favourite U2 song but joked, "My daughter nearly had an aneurysm when I told her who I was meeting with."