Security concerns have delayed plans to transfer Guantanamo Bay detainees to Yemen, challenging US President Barack Obama's efforts to close the prison.

White House officials said the threat posed by a Yemeni-based al-Qaida offshoot meant that sending the inmates to the Middle East nation was not an option for the foreseeable future, although that could change should conditions improve.

Obama lifted his self-imposed ban on repatriating prisoners to Yemen in May 2013, but his administration has yet to use that authority.

In a speech on Tuesday, he said: "Since I've been President, we've worked responsibly to cut the population of Gitmo in half. Now it is time to finish the job. And I will not relent in my determination to shut it down. It is not who we are. It's time to close Gitmo."

The administration is working on agreements with third countries willing to take Yemenis who are clear to leave the US prison in Cuba. Nearly two-thirds of the remaining 122 detainees are from Yemen, including 47 of the 54 who have been approved for transfer.

Meanwhile, the navy captain in charge of the naval station at Guantanamo Bay has been fired and is being investigated in connection with an alleged affair with a woman on the base and the recent death of her husband, US officials said on Wednesday.

AP - Xinhua