Senegalese troops have entered The Gambia to ensure Adama Barrow assumes power as the country’s new president, a Senegalese army spokesman has said.
It comes shortly after Mr Barrow took the oath of office at The Gambia’s embassy in the capital of Senegal.
He has been recognised internationally. But strongman Yahya Jammeh has refused to quit and is backed by parliament.
West African leaders have threatened to remove Mr Jammeh by force. The UN has backed their support for Mr Barrow.
The 15-member Security Council stressed on Thursday that this should be pursued “by political means first”.
Senegalese army spokesman Col Abdou Ndiaye was quoted by news agencies as saying the country’s troops entered The Gambia on Thursday afternoon.
Nigeria said earlier in the day that its “armed reconnaissance air force are over Gambia”, AFP reports.
“They have the capacity to strike,” Nigerian Air Force spokesman Ayodele Famuyiwa told the news agency.
West African military forces have made it clear they are ready to enforce a transfer of power in the country, a popular beach destination among European holidaymakers.
Mr Barrow took oath at the Gambian embassy in Dakar.
In his inauguration speech, he ordered all members of The Gambia’s armed forces to remain in their barracks.
“Those found illegally holding arms will be considered rebels,” he warned.
Western ambassadors to Senegal, the UN envoy for West Africa and officials from the regional bloc Ecowas (Economic Community of West African States) attended the ceremony, while hundreds of Gambian expatriates gathered outside the compound.
At the scene – Thomas Fessy, BBC News, Banjul, The Gambia
People have been following the inauguration of Adama Barrow live on Senegalese TV, which many receive here in The Gambia.
Few people have come out to celebrate, timidly chanting the name of Mr Barrow or waving at the cars driving by.
Tension is still running high, as people are very much aware that the political crisis is not over.
Banjul feels like a ghost town. Even the usually busy thoroughfares of Serekunda, on the outskirt of the capital, are deserted.
Many say the military remain – like Yayha Jammeh – unpredictable.
But in a sign that parts of the security forces may switch sides, I have met five police officers standing outside their station, relaxed and visibly happy.
I asked how things were going, and one of them replied with a smile “everything is alright, change is good.”
Meanwhile, Mr Jammeh’s term in office has been extended for three months by a two-thirds majority in parliament. Some experts say he still has a legitimate claim to be called the country’s president.
Gambian Information Minister Sidie Njie told the BBC on Thursday that Mr Jammeh would not leave his office.