Sameh Shoukry (centre) reads a document next to Chairperson Moussa Faki Mahamat (left) in Niamey on Thursday. AFP

African leaders will meet today (Sunday) in Niger for the African Union (AU) summit, to sign a landmark free trade agreement, and to discuss looming security and migration crises on the continent.

In a "historic" moment for the 55-member bloc, according to its chairman Moussa Faki, heads of state will officially launch the African Continental Free Trade Area (AfCFTA) at the two-day summit in Niamey, the Nigerien capital.

The agreement comes after 17 years of tough negotiations, and was formalised at the end of April when the agreement had crossed the launch threshold, which required ratification by at least 22 countries.

Nigeria, Africa's most populous country and largest economy, announced this week it would after all join the pact in Niamey, having unexpectedly pulled back from the agreement last year.

Nigeria's chief trade negotiator, Ambassador Chiedu Osakwe, said President Muhammadu Buhari would sign the landmark agreement, "opening Africa up to abundant opportunities.

"We weren't dragged into this, we are a leading advocate," Osakwe told AFP. "But it is about assessing how to make it work for Nigeria and indeed the continent."

State trade ministers agreed the zone should be operational from July 2020, AU Trade and Industry Commissioner Albert Muchanga told AFP, as countries needed time to adapt to the agreed changes.

An official start date will be agreed by heads of state Sunday -- with only Benin and Eritrea still to sign the agreement.

There are still key issues that need to be ironed out however, such as setting common criteria to determine rules of origin for traded products.

Amaka Anku, Africa analyst at Eurasia group, described the deal as a positive step but said the AfCFTA was still "a long way from taking off".

The AU estimates that implementing the AfCFTA will lead to a 60-percent boost in intra-African trade by 2022.

At the moment, African countries trade only about 16 percent of their goods and services among one another, compared to 65 percent with European countries.