Abuja - Nigeria's Acting President Goodluck Jonathan sacked the country's national security adviser (NSA) on Monday, in a further sign he is asserting his authority after the return of ailing President Umaru Yar'Adua.

Jonathan replaced Sarki Mukhtar, a powerful figure who was among the inner circle of Yar'Adua aides, with Aliyu Gusau, a retired northern general seen as a potential presidential candidate in elections due by April next year.

The move comes almost two weeks after Yar'Adua's unexpected return from a Saudi hospital, still too frail to rule, raised fears of a debilitating power struggle in Africa's most populous nation, a major oil producer of 140 million people.

Gusau, who served as national security adviser to former President Olusegun Obasanjo, was the main contender alongside Yar'Adua to be the ruling People's Democratic Party (PDP) candidate in 2007 elections, coming second in the primaries.

He remains a powerful kingmaker in the PDP.

Gusau has been tipped as a candidate. He could take over as vice president to Jonathan for the rest of the unexpired term should Yar'Adua die or resign, paving the way for him to be the ruling party's presidential nominee in the next elections.