Senegalese troops have entered The Gambia in support of Adama Barrow, who was sworn in as president on Thursday (local time) after winning last month's election.
Mr Barrow took the oath of office at the Gambian embassy in Senegal's capital, Dakar, and his legitimacy has been recognised internationally.
But the strongman he defeated, Yahya Jammeh, is refusing to quit and is supported by Gambia's parliament.
West African leaders have threatened to remove him by force if necessary.
The 15-member UN Security Council has given them its backing, while stressing that a political solution should be attempted first.
Nigeria deployed reconnaissance aircraft over The Gambia on Thursday as part of the mission, warning that it was ready to strike militarily.
The tiny country has been clouded by uncertainty for weeks. Thousands of Gambians have sought refuge in Senegal while tourists broke off their holidays to return home.
How dangerous is the situation?
A convoy of heavily armed Senegalese soldiers, in full battle dress, could be seen heading for the Gambian border.
They are backed by Nigerian air and naval power, as well as other troops from Ghana.
A Senegalese army spokesman, Colonel Abdou Ndiaye, said troops had encountered no resistance and were heading for Banjul.
"It is already war. If we find any resistance, we will fight it. If there are people who are fighting for the former president, we will fight them. The main goal is to restore democracy and to allow the new elected president to take over."
However, Gambian army chief Ousman Badjie said his troops would not fight Senegalese forces because the dispute was "political".
"I am not going to involve my soldiers in a stupid fight," he said.
"I love my men."
In his inaugural speech, President Barrow ordered all members of the Gambian armed forces to remain in their barracks.
Any found found illegally bearing arms would be considered "rebels", he said.
Why is Mr Jammeh refusing to go?
After first accepting defeat he reversed his position and said he would not step down.
He declared a 90-day state of emergency, blaming irregularities in the electoral process.
The electoral commission accepted that some of its early results had contained errors but said they would not have affected Mr Barrow's win.
Mr Jammeh has said he will stay in office until new elections are held.
Retaining power would also ensure him against prosecution for alleged abuses committed during his rule.