ABUJA - Nigerian President Umaru Yar'Adua is being treated in Saudi Arabia for acute pericarditis and the vice president has assumed presidential powers in his absence, Yar'Adua's spokesman said on Thursday.

Yar'Adua, 58, went to Saudi Arabia suddenly on Monday for medical checks after complaining of severe chest pains and his office said it was not clear when he would return home.

Pericarditis is an inflammation of the membrane surrounding the heart that can restrict normal beating.

Vice President Goodluck Jonathan has assumed presidential powers in the leading sub-Saharan oil producer and OPEC member nation during Yar'Adua's treatment, presidential spokesman Olusegun Adeniyi told Reuters.

Yar'Adua has travelled to Saudi Arabia in the past for treatment for a chronic kidney problem, raising questions about whether he will be fit enough to stand for a second term in 2011 elections.