LAGOS, Nigeria (CNN) -- Nigeria's House and Senate approved a resolution Tuesday to install the country's vice president as head of state until ailing President Umaru Musa Yar'Adua recovers enough to resume his duties.

But the Executive Council, made up of government ministers, must still approve the resolution before Vice President Goodluck Jonathan can become acting president.

The Executive Council is likely to discuss the matter at a regularly scheduled meeting Wednesday.

The absence of Yar'Adua, who left Nigeria for medical treatment in Saudi Arabia in November, has created a power vacuum in Africa's most populous country. It also sparked demonstrations in the nation's capital, Abuja, where protesters demanded a constitutional order on his absence and evidence about his true state of health.

The president is being treated for acute pericarditis, an inflammation of tissue around the heart. His illness was diagnosed in November after he complained of chest pain following prayers at a mosque in Abuja. He was taken to King Faisal Specialist Hospital and Research Centre in Jeddah, where he had his last checkup in August, his doctor said.

He said at the time he did not intend to resign while in Saudi Arabia, but no further news came from the president for almost two months until he was interviewed by the BBC from his hospital bed January 13.

In a frail voice, Yar'Adua assured his countrymen that he was alive and getting better, and intended to return to power soon.

Nigeria's Senate had expressed concern "that there had been no formal communication to the National Assembly" -- as required by the nation's constitution -- "(to) empower the vice president to act and perform the functions of the president," Voice of Nigeria reported.

But the country's attorney general has said the president did not need to write such a letter.

In addition to internal discord over its missing president, the oil-rich nation has faced some international heat after one of its citizens allegedly attempted to blow up a U.S.-bound flight on Christmas Day.

Nigeria was then cited as a country from which enhanced security measures would be required for passengers on U.S.-bound flights. A senior U.S. administration official said Nigeria fell into the U.S. Transportation Security Administration category of "state sponsors of terrorism or other countries of interest."

-- CNN's Christian Purefoy contributed to this report.

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