Gambia's Vice President Isatou Njie-Saidy has quit, Al Jazeera has learned, amid rising political tensions as Yahya Jammeh refuses to step down as president despite losing a December election.

Saidy, who had been in the role since 1997, is the highest level official to abandon Jammeh's camp in his standoff with opposition leader Adama Barrow, who won the election.

Jammeh, who lost a December 1 presidential vote to Barrow, has refused to leave office despite international pressure and a threat by leaders of the Economic Community of West African States, or ECOWAS, to enforce his election defeat.

Barrow - who fled to Senegal earlier in the week - has pledged to go ahead with his inauguration on Thursday.

The president-elect on Thursday tweeted that he would be holding the inauguration ceremony at the Gambian embassy in Dakar, the Senegalese capital.

Jammeh's whereabouts were unknown on Thursday, hours after a last-minute failed attempt by Mauritanian President Mohamed Ould Abdel Aziz to convince him to give up the presidency.

Troops from Senegal, Nigeria and Ghana on Thursday remained in position in Senegal in case a military intervention becomes necessary.

Sidi Sanneh, the former Gambian foreign minister, told Al Jazeera: "There is a certain faction in the [Gambian] army, who decided that they will not fight a losing battle [against the intervening troops] … As far as we are concerned, we don’t expect much of a resistance from that end of the military establishment."

He added: "However, there is another faction, the favourite of Jammeh, who enjoyed the fruits of the 22 years of dictatorship. We expect them to put up a fight. How much of a fight? That is still up in the air."