Senegal's forces are at the Gambian border and will enter at midnight if veteran Gambian President Yahya Jammeh refuses to leave power, Colonel Abdou Ndiaye, a spokesman for the Senegalese army, told Reuters on Wednesday.
Jammeh, who lost a Dec. 1 election to opposition leader Adama Barrow, said he would not step down, citing irregularities in the vote. His official mandate was due to end at midnight GMT.
"We are ready and are awaiting the deadline at midnight. If no political solution is found, we will step in," Ndiaye said.
The Nigerian Air Force said it had deployed to Senegal in case of need. Nigeria is part of the West African bloc ECOWAS, which has threatened Jammeh with sanctions or intervention if he does not step down.
Senegal's statement raises the prospect of armed confrontation between forces loyal to the president who has ruled Gambia for 22 years and Senegal, which surrounds the tiny country on three sides.
Authorities took steps to shore up Jammeh's authority this week. Jammeh declared a state of emergency on Monday, while on Tuesday the National Assembly passed a resolution to enable him to remain in office for three months.
Gambia has had only two rulers since independence in 1965. Jammeh seized power in a coup and his government has gained a reputation among ordinary Gambians and human rights activists for torturing and killing opponents.
Senegal circulated a draft resolution to the 15-member U.N. Security Council that would give "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".
The draft, seen by Reuters, would endorse the decision of ECOWAS and the African Union to recognise Barrow. It also called on Gambia's security forces to protect lives and property and serve the elected authorities.
It was not immediately clear when Senegal planned to put the draft resolution to a vote. U.N. Security Council approval is not needed for an ECOWAS military intervention in Gambia if Barrow requests help, some diplomats said.
Mauritania's President Mohamed Ould Abdel Aziz arrived in Gambia for last-minute talks ahead of the deadline, Gambian state television said.
Refugees Flee Gambia
Few people expected Jammeh to lose the election and the result was greeted with joy by many in the country and by democracy advocates across the continent, particularly when Jammeh initially said he would accept the result and step down.
Barrow, who is in Senegal, was examining the implications of the assembly's resolution and the state of emergency, given the constitutional requirement for a handover and the need to maintain peace, his spokesman Halifa Sallah told Reuters.
Barrow could be sworn in at the Gambian embassy in Senegal, which is technically part of Gambian territory, diplomats said.
Jammeh's decision to backtrack has unleashed turmoil. At least eight ministers have also resigned from his government.
At least 26,000 people have fled Gambia to Senegal fearing unrest, the U.N. High Commissioner for Refugees (UNHCR) said on Wednesday, citing Senegalese government figures.
"Up until the night of the 16th there were 26,000 people ... The flow has increased sharply since then," said Helene Caux, regional information officer for the UNHCR. She said up to 80 percent were children accompanied by women.
"Senegal has informed us that they have the capacity to take in 50,000 refugees in the short term and is making plans to accept up to 100,000," Caux said.
"People are afraid, we don't know what's going to happen," said a restaurant worker who gave only his first name, Musa, at an eatery in the capital Banjul normally buzzing but now empty of tourists.
"We hope he (Jammeh) will leave so this will be over quickly and things can get back to normal," he said.
Tour operator Thomas Cook started flying nearly 1,000 holidaymakers home on Wednesday. It said on its website it was laying on extra flights in the next 48 hours to remove 985 package tour customers.
It was also trying to contact a further 2,500 "flight only" tourists in Gambia to arrange for their departure on the earliest available flight, it said in a statement.
Gambia's economy relies on one main crop, peanuts, and tourism. Its beaches are popular with European holidaymakers seeking a winter break.