LAGOS (Reuters) - The leading female candidate in Nigeria’s presidential election said on Thursday she had withdrawn from the race weeks ahead of the poll to help build a coalition to provide a viable alternative to the country’s two main parties.

Former government minister Obiageli Ezekwesili, co-founder of a group to raise awareness about more than 200 girls kidnapped by the militant Islamist group Boko Haram in 2014, said on Twitter that she had been in talks for three months with other candidates about a coalition.

Nigeria’s presidential election is scheduled to take place on Feb. 16. The main candidates in the race to head Africa’s top oil producing country are the incumbent, Muhammadu Buhari, and Atiku Abubakar, a former vice president who is representing the main opposition People’s Democratic Party (PDP).

“I have decided to step down from the presidential race and focus on helping to build a coalition for a viable alternative to the #APCPDP in the 2019 general,” said Ezekwesili in a tweet.

Ezekwesili was not considered to be among the frontrunners in the race. Analysts see Buhari and Abubakar as the only genuine contenders due to the financial power and patronage networks provided by their parties.

The Independent National Electoral Commission (INEC) later said it was “impossible” for a candidate to withdraw at this stage.

“The last day for withdrawal by candidates or replacement of withdrawn candidates by political parties was 17th November, 2018. Therefore, the deadline for Ezekwesili or any candidate in that category to withdraw or be replaced has passed,” said INEC spokesman Rotimi Oyekanmi.

Ezekwesili was the candidate of the Allied Congress Party of Nigeria, a little known party. Following her announcement the party said it would support Buhari’s re-election bid.

The PDP ruled from 1999, when civilian rule began, until Buhari’s 2015 election win with his All Progressives Congress (APC) party. The main contenders ahead of February’s poll have been in the public eye for decades - Buhari was a military ruler in the 1980s and Abubakar was vice president from 1999-2007.

Ezekwesili, a former government minister, is known more recently for her work as co-founder of ‘Bring Back Our Girls’, a campaign which seeks to raise awareness about some 270 girls who were kidnapped from their school in the northeast Nigerian town of Chibok in April 2014 by Boko Haram.

Many of the girls managed to escape in the hours following their abduction or have been released in the last few years, but about 100 are still missing and their condition is unknown.