The Gambia's President-elect Adama Barrow says he will be sworn in at the country's embassy in neighbouring Senegal, as regional forces massed at the border to force incumbent Yahya Jammeh to quit after his election defeat.

In messages posted on his social media accounts, Barrow said that the inauguration was going to take place as scheduled on Thursday in the Senegalese capital, Dakar.

I would like to inform you that the Inauguration Ceremony is going to take place at the Gambian Embassy in Dakar, Senegal. #Gambia pic.twitter.com/ofxlIpJChl — Adama Barrow (@adama_barrow) January 19, 2017

Al Jazeera's Nicolas Haque, reporting from Dakar, said representatives of West African heads of states were expected to attend the swearing-in ceremony, due to take place at 16:00 GMT.

"A very significant moment for the Gambian history," Haque said.

There was a heavy security presence at Gambia's Dakar-based embassy on Thursday afternoon. Embassy staff climbed on to the roof to replace the faded Gambian flag with a new one.

It was not clear how Barrow will travel to Gambia.

VP quits

Earlier on Thursday, sources told Al Jazeera that Isatou Njie Saidy, Gambia's vice president since 1997, had quit, becoming the highest level official to abandon Jammeh's camp in his standoff with opposition Barrow, who won last month’s presidential election.

"Saidy's resignation comes after a series with defections among Jammeh’s entourage," Haque said.

AL JAZEERA'S NICOLAS HAQUE, REPORTING FROM DAKAR IN NEIGHBOURING SENEGAL: Troops from Senegal, Ghana, Nigeria, Mali and Togo are at the borders of Senegal, waiting for a green light to intervene and unseat Yahya Jammeh, who according to the constitution is no longer the country's legitimate ruler. Banjul has been turned into a "ghost town'"with tens of thousands of people fleeing the capital and at least 26,000 crossing the border into Senegal. Meanwhile, Ousman Badjie, the chief of defence staff, has said that he will not put his men on the line and they will not fight or die for Yahya Jammeh.

"Eight cabinet members have resigned saying they no longer stand with Jammeh. But despite all these defections, Jammeh is still not willing to concede defeat."

Jammeh initially acknowledged Barrow as the winner of the December 1 vote, but later rejected the result stating irregularities.

Earlier this week, he declared a national state of emergency, and on Wednesday The Gambia's national assembly approved a resolution to extend his term by 90 days.

Jammeh, who has ruled The Gambia since a 1994 coup, is refusing to step down, despite international condemnation and a threat of a military intervention by West African countries to enforce his election defeat.

In and around Gambia's capital, Banjul, shops were closed and streets quiet, with tour operators evacuating hundreds of tourists from the small West African country's popular beach resorts.

Military operation

The United Nations Security Council was to vote later on Thursday on endorsing a West African military intervention as Senegal, Nigeria and Ghana dispatched hundreds of troops and fighter jets to Gambia's border with Senegal.

Senegal's army had said on Wednesday it would be ready to cross into its smaller neighbour, which it surrounds, from midnight.

"A military operation [is under way] with troops also from Ghana, Nigeria, Togo, Mali - they are all at the Senegal border and presenting a united front," Haque said.

A senior Nigerian military source told Reuters news agency that regional forces would only act once Barrow had been sworn in.

"What the Senegalese said about the midnight deadline was to put pressure on Jammeh. It was a show of muscle," a diplomat in the region told Reuters.

The UN said at least 26,000 people fearing unrest have fled to Senegal and tour operators have sent charter jets to fly hundreds of European holiday makers out of the country.