It was hoped the Sunday appointment of Mali's new civilian prime minister, Moctar Ouane, could lift sanctions imposed by neighbouring countries.
ECOWAS imposed sanctions on Mali soon after the junta led August 18 coup and said they would be lifted after civilian leaders were appointed during the transition period.
But the bloc of West African leaders may take aim at junta leader Colonel Assimi Goita being appointed vice president.
He was one of the junta leaders who took power and toppled president Keita last month.
Former defence minister Bah Ndaw was sworn in as interim president on Friday and after announcing a new prime minister, he appearing to meet those conditions.
But Nigeria's President Muhammadu Buhari's office said regional leaders "may confer again to discuss outstanding grey areas in the Mali political situation" after he met ECOWAS envoy and Nigerian ex-president Goodluck Jonathan.
The statement said Buhari told the envoy to present a formal report to the ECOWAS chairman, Ghana's President Nana Akufo-Addo, "who will then write us officially, and we then determine the next steps".
"According to the special envoy, the military leaders are yet to satisfy ECOWAS demand of a full civilian as vice president, and what his roles would be in government," the statement said.