BUJUMBURA, June 9 (Xinhua) -- Burundi's outgoing President Pierre Nkurunziza has died of a heart attack in a hospital in east-central Burundi, said a government statement issued here on Tuesday.

Nkurunziza watched a volleyball match in Ngozi Province in the afternoon of June 6, before being taken to a hospital in Karusi Province for treatment as he felt discomfort in the evening, the statement said.

His health condition improved on Sunday but surprisingly changed later and died at the age of 55, it said, adding that immediate intensive care was conducted by a multidisciplinary team of doctors with cardio-respiratory assistance for many hours but were failed to recuperate the deceased.

"Burundian government offers its sincere condolences to his family and all Burundians," said the statement.

The government also announced a seven-day national mourning from Tuesday.

Government Spokesman Prosper Ntahorwamiye announced the statement at the national television station.

Speaker of the Burundian National Assembly Pascal Nyabenda will lead the transition government until August 20, as stipulated by the Constitution.

The "unexpected death" of Nkurunziza came days of the Constitutional Court declared Evariste Ndayishimiye the winner of May 20's presidential poll, who is expected to be inaugurated on August 20.

Ndayishimiye, Secretary General of the ruling National Council for the Defense of Democracy-Forces for the Defense of Democracy, was in January elected as the party's presidential candidate after Nkurunziza announced that he would not seek reelection.

Nkurunziza was elected the president of Burundi in 2005, and was re-elected in 2010 and in 2015.