Updated 5 p.m. ET

West African troops have crossed the border with Gambia in an effort to uphold the result of the country's presidential election by force.

The winner of the Dec. 1 vote, Adama Barrow, was officially sworn in as president at the country's embassy in neighboring Senegal earlier this afternoon. But Gambia's longtime leader, Yahya Jammeh, has refused to quit power despite mounting regional and international pressure.

The battle for power between two men who both claim to be president of Gambia has plunged Africa's smallest country into uncertainty.

"A newly sworn in President Barrow appealed to neighboring countries for help," NPR's Eyder Peralta reported. "And now troops from Nigeria, Senegal and other countries have entered Gambia."

U.S. State Department spokesman John Kirby told reporters that the United States supports the military intervention by ECOWAS, the West African regional bloc. "We support it because we understand that the purpose is to help stabilize a tense situation and to try to observe the will of the people of the Gambia," he said.

U.S. forces are not involved, Kirby added, and U.S. citizens in Gambia are advised to shelter in place.

In Barrow's inaugural speech, he emphasized the historic nature of his win: "This is the first time since The Gambia became independent in 1965 that Gambians have changed their government through the ballot box."

Jammeh initially accepted the result of the Dec. 1 election. But a week later, he dramatically reversed course and claimed that the election results are illegitimate. He later announced a state of emergency, and on Wednesday, the parliament extended his term for three months.

"Exceptional circumstances have compelled me to be sworn here today," Barrow said at the televised ceremony in Dakar. "My right as a winner to be sworn in and assume the office of president is constitutionally guaranteed and irreversible. ... I therefore call on all civilians and military personnel of the state to support my presidency, since it is built on a constitutional foundation."

Barrow, a businessman, vowed to pursue "comprehensive reforms." He fled to Senegal for his own protection — like thousands of other Gambians in recent weeks.

Meanwhile, the U.N. Security Council has backed a resolution in support of the new leader. The council voted unanimously to adopt the resolution, which "expresses its full support to the ECOWAS (Economic Community of West African States) in its commitment to ensure, by political means first, the respect of the will of the people of The Gambia as expressed in the results of 1st December elections."

The resolution also stressed that Gambian defense and security forces have a "duty and obligation to place themselves at the disposal of the democratically elected authorities." T

The U.N. Secretary-General said in a statement that he told Barrow he fully supports "his determination, and ECOWAS's historic decision, with the unanimous backing of the Security Council, to restore the rule of law in The Gambia so as to honour and respect the will of the Gambian people."

The African Union has stated that its recognition of Jammeh's authority would end with the end of his term on Wednesday, at the stroke of midnight.

On the streets of Gambia, celebrations have erupted in support of the new leader, as Eyder reports.

"I'm just really happy about Gambians celebrating in this fashion. And I really hope this peace and stability continues," Omar Jallow, a lawyer in Gambia's capital Banjul, tells Eyder. He adds that he "knows there could be clashes between the foreign troops and Jammeh supporters."

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