Gambian President Adama Barrow has taken the oath of office in neighbouring Senegal, while the country's defeated longtime ruler refuses to step down from power, deepening a political crisis.

Barrow was inaugurated on Thursday in a hastily-arranged ceremony at Gambia's embassy in the Senegalese capital, Dakar.

"This is a day no Gambian will ever forget in a lifetime," Barrow said in a speech immediately after being sworn in.

The small embassy room held about 40 people, including Senegal's prime minister and the head of Gambia's electoral commission.

Also at the event were officials from West Africa's regional bloc, ECOWAS, which is threatening a military intervention to force outgoing president Yahya Jammeh, who lost a December 1 election, to step down.

The UN Security Council was set to vote later on Thursday on a draft resolution endorsing a military intervention by ECOWAS members to remove Jammeh.