A level delegation which includes UN West Africa envoy Mohamed Ibn Chambas, Nigerian President Muhammadu Buhari, and Liberian leader Ellen Johnson Sirleaf in her capacity as head of the west African ECOWAS bloc, have set out to meet President Yahya Jammeh of Gambia
Also, the Gambian president-elect Adama Barrow said on Monday longtime leader Yahya Jammeh should leave power immediately, as the diplomatic community threatened "draconian" measures if he failed to leave.
In a speech on Friday, Jammeh said he would reject the results of a December 1 election he lost "in totality", and called for new polls after a vote recount showed his defeat was narrower than first announced.
"I think he should step down now," Barrow told AFP. "He has lost the election, we don't want to waste time, we want this country to start moving."
Barrow is due to welcome a heavyweight group of African heads of state and UN representatives to persuade Jammeh to leave, following the ruling party's declaration on Saturday it will challenge the results in court.
If Jammeh and the delegation did not reach an agreement, west African states "will contemplate more draconian decisions", top ECOWAS official Marcel Alain de Souza told French radio station RFI.
Jammeh's swift concession of defeat had initially stunned observers and led to celebrations across the country.
But Jammeh's party complicated what was expected to be a peaceful handover when it announced on Saturday that the legal complaint would be filed with the Supreme Court.
Barrow has urged Jammeh to meet the heads of state when they arrive but could not confirm that the longtime leader would be present, saying that he had not spoken to him since December 2.
"I urge him and advise him to meet the international community. They are partners to The Gambia and The Gambia is a signatory to those institutions, so we have to give them that respect," he said.
Outgoing Ghanaian President John Mahama is also expected to join the group on Tuesday, a regional example of a leader who just lost his own election but is nonetheless expected to hand over power without issue.
The president-elect said that Jammeh's threat to use the courts over a vote recount was baseless and that he had no power to appoint to the Supreme Court new judges who would be necessary to hear the case. The court has sat dormant since May 2015.
"We don't have time to fight again. The Supreme Court wasn't existing for the last one year. There are a pile of cases that are waiting... but he doesn't care about it," Barrow told AFP.
Late on Monday the Gambian Bar Association, an organisation representing the country's most powerful lawyers, described the court challenge as "tantamount to treason".
It condemned the "illegitimate and destabilising actions of the outgoing President" and called for a boycott of the courts until Jammeh handed over power.
The Bar Association said there was "no legitimate legal mechanism available in The Gambia to hear and determine the election petition", as he would have to stuff the court with his own appointees.
A readjustment of the votes counted in the election was made on Monday last week, reducing numbers of ballots for all three candidates but ultimately confirming Barrow's victory.
The Chairman of the Independent Electoral Commission Alieu Momar Njie told AFP on Monday "we were not covering up any mistake".
"(Last) Monday we held a meeting with all the parties involved. They all accepted the result," he said.
Jammeh has led the tiny sliver of a nation of just under two million people for 22 years since taking power in a coup.