The US National Security Agency has admitted its analysts intentionally engaged in surveillance in violation of legal restrictions.

In a statement to Bloomberg News, the NSA acknowledges "very rare instances of wilful violations of NSA’s authorities have been found" over the past decade.

One NSA official says they were the result of over zealous NSA employees eager to prevent any repeat of the 9-11 terrorist attacks. The statement contradicts earlier assertions by the Obama administration officials and lawmakers that no deliberate violations have occurred.