The office of Nigerian President Goodluck Jonathan is responding harshly to a letter from former President Olusegun Obasanjo, who strongly urged Mr. Jonathan not to seek re-election in 2015.
In a statement, presidential adviser Reuben Abati acknowledged the letter was received, but says it is "highly unbecoming, mischievous and provocative" that the letter was leaked to the media.
He says the president has told aides not to discuss the content of the letter, but adds Mr. Jonathan will eventually respond to what the adviser calls the "reckless, baseless" charges against him.
Several news organizations have published excerpts of the 18-page letter. In it, Mr. Obasanjo writes that it would be a "fatally and morally flawed" decision for Mr. Jonathan to be a candidate in 2015.
The former president criticizes Mr. Jonathan's leadership and said he has allowed large-scale corruption in Nigeria, which is Africa's most populous nation and its biggest oil producer.
He also suggests Mr. Jonathan may be inadvertently dividing the country along religious lines.
Some Nigerians say Mr. Jonathan, a Christian from southern Nigeria, has broken an unwritten rule the presidency should rotate between the mostly Christian south and the mainly Muslim north.
Mr. Jonathan initially came to power in 2010, after the death of President Umaru Yar'Adua, a Muslim, after less than three years in office.
He was elected to his first full term the following year, in an election marred by violence and allegations of vote-rigging.
Mr. Obasanjo served as Nigeria's president from 1999 to 2007. He was previously a mentor and supporter of Mr. Jonathan, but relations between the two men have soured in recent years.
President Jonathan has faced a number of recent challenges to his leadership, including defections within the ruling People's Democratic Party.
In November, five powerful state governors who had left the People's Democratic Party announced they had joined the opposition All Progressives Congress Party.
The move bolsters the opposition's strength in parliament and could position it to mount a strong challenge against the People's Democratic Party in the upcoming elections.
The People's Democratic Party has been in power for 15 years, but has struggled with internal conflicts, especially since Mr. Jonathan's election.