Banjul - Four African heads of state were due in The Gambia on Tuesday morning with a mission to persuade President Yahya Jammeh to leave office after his defeat at the ballot box.Jammeh's party has vowed to challenge the December 1 vote result in court, leading to an avalanche of international condemnation and multitude of calls for him to cede power to opponent Adama Barrow, who was officially declared the winner.Jammeh is expected to meet Nigeria's Muhammadu Buhari, Liberian leader Ellen Johnson Sirleaf, Sierra Leone's Ernest Bai Koroma and Ghana's outgoing President John Mahama this morning.The heavyweight delegation of west Africa's biggest hitters, who have significant ties to The Gambia, will be joined by United Nations West Africa envoy Mohamed Ibn Chambas.The African leaders will then hold separate talks with Barrow, several sources told AFP.Banjul-based diplomats say Buhari in particular has long been annoyed by Jammeh's provocative behaviour and disdain for protocol.'Draconian decisions'Up until now the president of the tiny country of fewer than two million people may have exasperated his peers but has never threatened peace in the sub-region, a situation that has dramatically shifted since Jammeh's move to void the election."It is unacceptable that there is an election and one person turns down the result," Liberia's information minister Eugene Nagbe told AFP on Tuesday. "The message of President Sirleaf and her delegation to Jammeh will be that he accepts the result and gives way to smooth transition."If Jammeh and the delegation did not reach an agreement, west African states would "contemplate more draconian decisions", a top official with the regional Ecowas bloc headed by Sirleaf told French radio station RFI late Monday.Streets from the airport were quiet as Gambians awaited the leaders' arrival, but some parents kept their children home from school as a precaution.President-elect Barrow has told AFP he wants Jammeh to step down "now", though the longtime leader has the legal right to stay in office until mid-January.The African Union has also promised to dispatch its own delegation as soon as possible to aid the transfer of power, while a statement released Monday said it rejected "any attempt to circumvent or reverse the outcome of the presidential election."Court decision?Jammeh, who took office in a coup, has led The Gambia for 22 years.Meanwhile it was unclear whether Jammeh's party would file a complaint with the Supreme Court on Tuesday, thought to be constitutionally the last day possible to contest the election result.A group of the country's most influential lawyers has said there is "no legitimate legal mechanism available in The Gambia to hear and determine the election petition", as Jammeh would have to stuff the court with his own appointees.The legal body has lain dormant since May 2015 as Jammeh himself sacked many of its judges.A readjustment of the votes counted in the election was made on Monday last week, reducing the number of ballots for all three candidates but ultimately confirming Barrow's victory.Overnight the US ambassador to the UN, Samantha Power, warned that The Gambia faced "a very dangerous moment", citing reports that some military officers have sided with Jammeh in the standoff.Jammeh has led The Gambia for 22 years since taking power in a coup.