Britons begun to fly back from The Gambia as the clock ticks past a midnight deadline for military action in the West African nation.
Senegalese forces say they will cross the border if the president refuses to step aside.
Nigeria's air force has also deployed fighter jets and surveillance planes to Senegal, which borders The Gambia.
Mauritanian president Mohamed Ould Abdel Aziz flew to Senegal to meet with that country's president Macky Sall, according to a Senegalese presidential source.
With fears that airports could shut at short notice, holiday companies are urgently flying thousands of Britons home.
The first UK tourists to arrive at Manchester Airport told of a "chaotic" scramble to get home.
Sara Wilkins, from Church Stretton, Shropshire, said: "It was a nightmare at the airport - people were crying and panicking. It was just chaos."
Karl Degnan from Nottingham said: "The communication at the hotel was very poor. I got up this morning to get some breakfast and was just told we've got to pack, we've got to go."
It comes after Gambian President Yahya Jammeh declared a state of emergency after refusing to hand over power following his loss in last month's election.
Adama Barrow, a former Argos security guard in London, was declared the unexpected winner.
Senegal had said regional troops would take action if Mr Jammeh, who has ruled for more than two decades, did not step down when his mandate ended at midnight.
"We are ready and are awaiting the deadline at midnight. If no political solution is found, we will step in," said Colonel Abdou Ndiaye, a Senegalese army spokesman.
The Nigerian air force is ramping up the pressure.
It says it has moved "200 men and air assets comprising fighter jets, transport aircraft, light utility helicopter as well as intelligence, surveillance and reconnaissance aircraft" to Dakar in Senegal.
Ghana has also pledged to contribute to the regional force.
British holidaymakers have been urgently heading home after the Foreign Office warned of the potential for military action.
There were some emotional scenes as tourists hugged locals and staff at hotels and boarded coaches to the airport.
Thomas Cook said it had 985 package tourists and 2,500 "flight-only" customers in the West African country.
It expects to have brought them all back - on 16 flights - by the end of Friday.
When Mr Jammeh lost the December election he said he would move aside but then refused, claiming there were irregularities in the voting.
Human rights activists have accused him of torturing and killing his opponents, including journalists.
Senegal has also been trying to win the support of the UN Security Council for potential military action.
According to Associated Press, it circulated a draft resolution reiterating the Council's "full support to the ECOWAS in its commitment to take all necessary measures to ensure the respect of the will of the people of the Gambia".