The 15-nation regional bloc known as ECOWAS had issued the ultimatum to Mali's military leaders after they initially proposed remaining in power for three more years.
In recent days, the junta - which seized power in a coup in August - has cut that timeline in half.
But it is still longer than the year set by ECOWAS.
Another possible disagreement is over who will lead Mali's transition. The coup leaders say the interim president chosen to oversee the transitional government could come from the military, but the regional leaders have said the leader must be a civilian.
These issues are expected to be at the top of the agenda on Tuesday at the summit, which includes presidents from six countries in the regional bloc.
The gathering is taking place in Ghana, whose president is the newly chosen ECOWAS chairman.
Ghanaian President Nana Akufo-Addo said in his opening remarks that "the circumstances of life in Mali today require that closure be brought to the matter now."
There has been widespread concern that the ongoing political upheaval in Mali will set back efforts to contain the country's growing Islamic insurgency.
After a similar coup in 2012, Islamic extremists took advantage of a power vacuum and grabbed control of major towns in northern Mali.
Only a 2013 military intervention led by former colonial power France pushed extremists from those cities, and the international community has invested more than seven years into the fight against extremism there.
"The terrorists are taking advantage of the situation in Mali to flex their muscles even more," Ghana's president warned.
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