The French, largely unfancied ahead of the finals in Brazil, impressed in the group stages, seeing off Group E rivals Honduras and Switzerland 3-0 and 5-2 respectively.
A 0-0 draw in their final match with Ecuador secured their place as group winners, setting up Monday's last-16 tie with Nigeria in Brasilia.
Sagna, who made his first appearance of the tournament in Wednesday's stalemate, gave notice of the problems the African outfit will pose Didier Deschamps' men.
"They are the best African team at the moment," the Manchester City-bound full-back said.
"They have very good players. Some play in the biggest European clubs, especially in England.
"They will fight and they have a lot of qualities. But we want to concentrate on ourselves.
"As players, we aspire to go as far as possible, to win trophies.
"It would be a failure not to win the trophy. Initially, we wanted to qualify from the group. Now, we want to go step by step.
"There are 11 players who will start, but the 23 are important. The atmosphere is very important and we try to keep this dynamic."
