The African Union has urged Gambia's president to facilitate a "peaceful" transfer of power after he said no longer accepts defeat in the country's election.
It has called a statement by President Yahya Jammeh rejecting his loss in December 1 polls "null and void" as he had already conceded defeat to his rival.
Jammeh lost to opposition leader, Adama Barrow - a former Argos security guard - in last week's presidential election.
Speaking on state television, Jammeh said he now rejected the poll results, blaming voting irregularities for his about-turn. He is calling for a repeat election.
"Our investigations reveal that in some cases voters were told that the opposition has already won and that there was no need for them to vote and, out of anger, some of them returned home," he said.
"I hereby reject the results in totality," he said. "I will not accept the results based on what has happened."
On Saturday, the plane of the Economic Community of West African States (ECOWAS) chair Ellen Johnson Sirleaf was denied access to Gambia's Banjul airport, Senegal's foreign minister said.
The reason for the denial was unclear, as was the intended timing of the plane landing, but ECOWAS had been hoping to play a role in creating a smooth transition of power in the Gambia.
Meanwhile, the US government has condemned Jammeh's rejection of the election result, calling it an attempt to remain in power illegitimately.
"The people have spoken and it is time for Gambians to come together to ensure a peaceful transition to President-elect Barrow," said State Department spokesman Mark Toner.
Jammem wished Barrow 'all the best' after victory
Barrow's surprise win ended two decades of rule by Jammeh, who once said he would lead for "a billion years".
Following last week's results, Jammeh was filmed on state television calling the opposition candidate to wish him the best.
"You are the elected president of The Gambia, and I wish you all the best," Jammeh told Barrow at the time. "I have no ill will."
The Jammeh regime has long been accused of imprisoning, torturing and killing its opponents, according to human rights groups. He also has issued increasingly virulent statements against sexual minorities, vowing to slit the throats of gay men.