Gusau submitted his resignation letter to Jonathan, which the president accepted, on Thursday and a day later informed him he intended to enter the presidential race, his spokesman Adebisi Adekunle said, according to a Reuters report.

"I can confirm to you that the national security adviser has resigned," Adekunle said, adding Gusau intended to collect his presidential nomination form from the ruling People's Democratic Party (PDP) offices on Monday.

Gusau's move means Jonathan -- who had been expected to travel to New York for the U.N. General Assembly late on Saturday but postponed the trip at the last minute -- will face a more challenging contest at the PDP primaries in mid-October.

Jonathan formally launched his campaign on Saturday with a rally in the capital Abuja attended by more than two thirds of the country's powerful state governors.

As the incumbent, he holds the strings of power and can be judged by voters on his performance so far, including helping to cement an amnesty in the oil-producing Niger Delta and more recently unveiling a blueprint to solve the chronic power shortages that plague Africa's most populous nation.

But although he is the front-runner, his bid is opposed by some in the ruling party because of an unwritten agreement that power should rotate between the north and south every two terms.

Jonathan inherited the presidency when president Umaru Yar'Adua, a northerner, died this year during his first term, but he is a southerner and some in the People's Democratic Party (PDP) have said the next leader must be a northerner.