Senegalese troops are moving towards the Gambian border from two sides, witnesses confirmed, as pressure mounts for Gambia's president to step down.
President Yahya Jammeh lost an election on December 1 and initially conceded defeat, but later said voting irregularities invalidated the ballot.
His mandate expires on Thursday but Mr Jammeh insists he will stay in power.
President-elect Adama Barrow is vowing to go ahead with his inauguration.
The West African regional bloc known as ECOWAS has threatened to use force to remove Mr Jammeh if necessary.
Witnesses in the Kaolack region of Senegal to the north of Gambia, and in the Casamance region to the south of Gambia, reported seeing a build-up of Senegalese soldiers.
Private Senegalese radio station RFM also said that Nigerian military equipment had started arriving in the Senegalese capital, Dakar.