The Obama administration's top intelligence official says the government has conducted warrantless searches of Americans' communications as part of the National Security Agency's spy operations that target foreigners located outside of the U.S.

James Clapper acknowledged these searches in a letter to Congress, but he did not say how often they occur.

The searches were authorized by a secret surveillance court in 2011, but it had been unclear whether any such searches on Americans had been conducted.

Some lawmakers and civil liberties advocates say these searches are unconstitutional. But the Obama administration contends that the searches are legal because they are searching information they lawfully obtained.