NSA chief Keith Alexander and deputy to depart within months

The director of the US National Security Agency and his deputy are expected to depart in the coming months, a development that could give President Barack Obama a chance to reshape the eavesdropping agency.

Army General Keith Alexander's eight-year tenure was rocked this year by revelations contained in documents leaked by former NSA contractor Edward Snowden about the agency's widespread scooping up of phone, e-mail and social-media data.

Alexander has formalised plans to leave by March or April, while his civilian deputy, John "Chris" Inglis, is due to retire by year's end.

One leading candidate to replace Alexander is Vice-Admiral Michael Rogers, commander of the US Navy's 10th Fleet and US Fleet Cyber Command. The 10th Fleet and Fleet Cyber Command both have their headquarters at Fort Meade, Maryland, between Washington and Baltimore. The NSA is also based at Fort Meade.

Officials said there had been no final decision on selecting Rogers and other candidates may be considered.

NSA spokeswoman Vanee Vines said Alexander planned to leave office in the spring after three extensions to his tenure, and the process for picking his successor was still under way.

"This has nothing to do with media leaks, the decision for his retirement was made prior; an agreement was made with the (secretary of defence) and the chairman for one more year - to March 2014," Vines said.

Alexander has served as NSA director since August 2005, making him its longest-serving chief. He also serves as commander of a related military unit, the US Cyber Command.

Alexander, who has vigorously defended the NSA's activities as lawful and necessary to detect and disrupt terrorist plots, has said previously that he planned to leave in the first half of 2014.

Inglis, who began his NSA career as a computer security scientist, has been the NSA's second-ranking official since 2006.

The departures will give Obama an opportunity to decide whether the NSA and Cyber Command should have separate leaders.