Parliament empowered Vice-President Goodluck Jonathan on Tuesday to take over for the ill president of oil-rich Nigeria, whose absence has stoked unrest in Africa's most populous country.

Nigerian President Umaru Yar'Adua welcomes U.S. Secretary of State Hillary Clinton at the state house in Abuja, Nigeria on Aug. 12, 2009. (Sunday Alamba/Associated Press)

Both the House of Representatives and the Senate passed measures calling on Jonathan to act as president and commander in chief until President Umaru Yar'Adua returns from Saudi Arabia, where he has been receiving medical treatment for a heart condition since November.

Yar'Adua's absence has caused a ceasefire with militants in the oil-rich delta to unravel and left no one formally in charge of the nation of 150 million, which has a long history of coups and military dictatorships. The latest crisis has drawn international attention, with U.S. Secretary of State Hillary Clinton and European leaders calling on the nation to follow its constitution.

Newspapers began worrying about possible coup scenarios as Yar'Adua's absence grew longer. But military leaders said several weeks ago they had no ambitions to take power and would respect the constitution.

It was unclear when Jonathan would be sworn in as president.

The National Assembly's empowering of Jonathan may set the stage for new legal battles and power struggles. The motion would allow Yar'Adua to reassume the presidency if he returns to the country healthy enough to lead the nation. However, many have worried the president may be too seriously ill to serve again, throwing into question who will lead the ruling party in the 2011 presidential election.

Yar'Adua, who long has suffered from kidney ailments, left Nigeria for Saudi Arabia on Nov. 23. He was admitted to hospital the next day for what his physician says is acute pericarditis, an inflammation of the sac surrounding the heart.