Leaders from the West African nations of Guinea and Mauritania were in the Gambian capital of Banjul Friday in an attempt to convince defeated President Yahya Jammeh to cede power and leave the country or face military action.
The leaders gave Jammeh until midday local time to relinquish power, the 15-nation Economic Community of West African States (ECOWAS) said.
After the deadline was imposed, Jammeh asked for a four-hour extension to cede.
Guinean President Alpha Conde and Mauritania's President Mohamed Ould Abdel Aziz arrived together in Banjul on Friday.
ECOWAS Chairman Marcel Alain de Souza said Conde would offer Jammeh the chance to cede authority peacefully. "If that fails, we will bring him by force or by will," he said.
The West African regional force moved into Gambia Thursday evening, de Souza said. At least 20 military vehicles were at the border town of Karang on Friday morning.
The United Nations Security Council approved the military intervention and has recognized Adama Barrow as the new president.
Barrow took the oath of office Thursday at the Gambian Embassy in neighboring Senegal. He was to have been sworn in at the Gambian capital of Banjul.
WATCH: Gambia's President-elect Adama Barrow arrives for his inauguration in Senegal
US supports show of force
On Thursday, U.S. State Department spokesman John Kirby said Washington supports the West African force "because we understand that the purpose is to help stabilize a tense situation and to try to observe the will of the people of Gambia."
Kirby said the U.S. has no plans to send in any U.S. troops.
The Security Council unanimously backed a Senegalese-drafted resolution condemning "in the strongest possible terms" attempts to prevent a peaceful and orderly transfer of power.
Council President Olof Skoog, the Swedish ambassador, said he personally spoke to President Barrow to tell him he has the council's full support.
Council members Egypt, Uruguay and Bolivia stressed that the resolution in no way authorizes military force to install Barrow as president.
ECOWAS and the African Union Peace and Security Council have called in separate communiques for "all necessary measures" to be taken to respect the will of the Gambian people regarding the election outcome. In diplomatic language, that often means the use of military force.
The United Nations refugee agency, meanwhile, said Friday that tens of thousands of Gambians were fleeing the country amid the standoff.
Spokesman Babar Baloch told reporters in Geneva an estimated 45,000 people had reportedly fled to Senegal in recent days and said there is the possibility of many more to follow.
Before he was sworn in Thursday, a jubilant Barrow said his country's flag will "fly high among the most democratic nations of the world. The new era of Gambia is here at last. This is a day no Gambian will ever forget."
This is the first time since Gambia became independent in 1965 that Gambians have changed their government through the ballot box."
The celebrations in the streets of Banjul began slowly because of the uncertainty and the presence of Gambian security forces; but, the partying grew when it became clear the troops had no intention of using force.
Ballot box success
Barrow won the country's December 1 election. Jammeh, who once vowed to rule Gambia for "a billion years," initially accepted the results, but changed his mind, citing alleged voting irregularities.
He has refused to give up power, declaring a state of emergency and ramming through a now meaningless three-month extension of his mandate through the parliament.
Amnesty International and other major human rights groups accuse Jammeh of having little tolerance for dissent. They say he has killed or jailed many opponents. He also has threatened to murder homosexuals and once ordered the kidnapping more than 1,000 villagers accused of being witches.
WATCH: Gambians flee ahead of presidential showdown