A military commander with West Africa's regional bloc says its forces will "take action" at midnight unless a solution is found to Gambia's political crisis before then.
Speaking on Senegalese radio station RFM, Seydou Maiga Mboro of Niger declared that "all the troops are already in place" awaiting the deadline for President Yahya Jammeh to step down.
Witnesses in Senegal have reported seeing troops making their way towards Gambia's borders.
At about the same time on Wednesday, Senegal circulated a draft UN resolution to members of the Security Council on the actions in Gambia planned by the West African regional bloc known as ECOWAS.
The key paragraph in the draft, obtained by The Associated Press, reiterates the Security Council's "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 as expressed in the results of December 1 elections".
Despite losing in the December elections to opposition candidate Adama Barrow, Mr Jammeh has refused to cede power after 22 years at the helm.
President-elect Barrow has vowed to go ahead with his inauguration anyway on Thursday, and ECOWAS, the regional bloc, has pledged to use military force if necessary to remove him. Mr Jammeh seized power of Gambia in a 1994 coup.
As the crisis deepened, hundreds of beach-going tourists were evacuated from Gambia, a popular winter destination in West Africa for visitors especially from Britain, the former colonial power. The streets of the capital, Banjul, were largely empty by late afternoon on Wednesday. There was very little military presence seen, said residents.
The tiny West African nation facing a potential regional military intervention is completely surrounded by Senegal and the Atlantic Ocean. Late on Wednesday, witnesses reported seeing Senegalese soldiers in both the Kaolack region of Senegal to the north of Gambia, and in the southern Senegalese region of Casamance.
Mr Jammeh lost the December 1 election after more than 22 years in power following a coup. He initially conceded defeat but later said voting irregularities invalidated the ballot. His mandate expires on Thursday, and the president-elect is vowing to go ahead with his inauguration on Gambian soil though he has not offered any details.
"Those who resist peaceful change effective 12 midnight tonight shall face definite consequences, to their peril," said Mai Ahmad Fatty, Mr Barrow's special adviser, in a Facebook post in which he advised Gambians to stay indoors. "Anyone with firearms tonight shall be deemed a rebel, and will certainly become a legitimate target."
In addition to the Senegalese soldiers, private radio station RFM also reported the arrival of Nigerian military equipment in the capital of Dakar. In addition to Nigeria and Senegal, Ghana has also pledged to contribute to the regional force.