The U.N. Security Council on Thursday backed ECOWAS's efforts to ensure Barrow assumes power, and the United States said it supported the intervention.
U.N. Secretary-General Antonio Guterres in a statement pledged "his full support for his (Barrow's) determination, and ECOWAS's historic decision, with the unanimous backing of the Security Council, to restore the rule of law in The Gambia so as to honor and respect the will of the Gambian people."
ECOWAS and the African Union previously said they would recognize Barrow from Thursday, and nations including the United Kingdom and France were quick to congratulate him.
ECOWAS will send a team led by Guinea's president, Alpha Conde, and including the presidents of Liberia and Mauritania to Banjul on Friday, de Souza said.
If the mission succeeds, Jammeh will travel to Guinea before choosing a country of exile.
"It's out of the question that he stays in place. ... We propose that he leaves in an honorable manner and with respect," said de Souza, who added that regional leaders were open to the possibility of an amnesty as part of a deal.
It was unclear what Jammeh's next move would be. He has so far ignored pressure to step aside and offers of exile.
He now faces almost total diplomatic isolation and a government riddled by defections. In the biggest loss yet, Vice President Isatou Njie Saidy, who has held the role since 1997, quit on Wednesday.
In a statement released late on Thursday, Jammeh announced he was dissolving his government - a 19-member Cabinet, half of whose members had already resigned - and pledged to name a new one "in due course."
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West African military halt Gambia operation, issue Jammeh deadline