Khalifa, who was born in 1948, came to power in 2004 in the richest emirate Abu Dhabi and became the head of state.
He is expected to be succeeded as ruler of Abu Dhabi by his half-brother Crown Prince Sheikh Mohammed bin Zayed al-Nahyan.
Mohammed bin Zayed has been the de facto ruler of the U.S.-allied UAE, particularly after Khalifa suffered a stroke in 2014, following which he has rarely been seen in public.