Troops from a bloc of West African countries have entered Gambia in support of Adama Barrow, shortly after the new Gambian president called for international backing following his inauguration in neighbouring Senegal.

Longtime ruler Yahya Jammeh, who came to power in a 1994 coup, has refused to step down despite losing a disputed December 1 presidential election to Barrow, deepening a political crisis.

In a statement, Senegal's army said on Thursday that forces from ECOWAS, West Africa's regional bloc, had begun strikes as part of an operation aimed at upholding the result of last month's vote.

Colonel Abdou Ndiaye did not specify the type of strikes, but said "significant" land, air and sea resources had been made available .

Earlier on Thursday, Barrow, who had recently sought shelter in Senegal, took the oath of office in a hastily arranged ceremony at Gambia's embassy in the Senegalese capital, Dakar.

"This is a day no Gambian will ever forget in a lifetime," Barrow said in a speech immediately after being sworn in.

Not long after his inauguration, the United Nations Security Council unanimously adopted a resolution backing Barrow and called for a peaceful transfer of power.

"The people of The Gambia spoke clearly at the elections in December. They chose Adama Barrow to be their president. Their voice now needs to be heard and their will needs to be heeded by just one man," Peter Wilson, the UK deputy ambassador to the UN, said.

Earlier this week, Jammeh had declared a national state of emergency , while the parliament extended his term in office by 90 days. He has not been heard from since his mandate expired at midnight.

At least 26,000 people have fled Gambia for Senegal since the start of the crisis fearing unrest, the UN's refugee agency UNHCR said on Wednesday, citing Senegalese government figures.

'Freedom, hope'

In Dakar, the small embassy room held about 40 people, including Senegal's prime minister, the head of Gambia's electoral commission and officials from ECOWAS.

In his inauguration speech, Barrow called ECOWAS, the African Union and UN to "support the government and people of the Gambia in enforcing their will".

He also ordered Gambia's armed forces to remain in their barracks and called for "allegiance to the motherland".

Al Jazeera's Nicolas Haque, reporting from Dakar, said large crowds of young people had gathered outside the embassy shouting "freedom" and "hope".

"People who have spent their entire life under one leadership, one person, and they see in Barrow an opportunity for change".

Haque added, however, that the "situation is still precarious. As we understand, Jammeh is not ready to let go of Gambia; he is still in charge at least in the capital, Banjul."

Hundreds of West African soldiers had previously deployed to the Gambian border to back Barrow in a showdown with Jammeh.

READ MORE: Exiled Gambians ponder return to troubled homeland

Senegal's army had said on Wednesday that it would be ready to cross into its smaller neighbour, which it surrounds, from midnight.

"A military operation [is under way] with troops also from Ghana, Nigeria, Togo, Mali - they are all at the Senegal border and presenting a united front," Haque said earlier on Thursday .

Also on Thursday, sources told Al Jazeera that Isatou Njie-Saidy, Gambia's vice president since 1997, had quit, becoming the highest level official to abandon Jammeh's camp.

Haque said Saidy's resignation had come after a series of defections among Jammeh’s entourage.

"Eight cabinet members have resigned saying they no longer stand with Jammeh. But despite all these defections, Jammeh is still not willing to leave office," he said.

Jammeh, whose mandate expired at midnight, had initially conceded defeat but a week later contested the poll's results stating irregularities.

READ MORE: Thousands flee Gambia as crisis deepens

Jammeh has resisted strong international pressure for him to step down, but African nations have begun stepping away from him, with Botswana announcing on Thursday that it no longer recognised him as Gambia's president.

Jammeh's refusal to hand over power "undermines the ongoing efforts to consolidate democracy and good governance" in the Gambia and Africa in general, it said.

Earlier this month, the African Union announced that it would no longer recognise Jammeh once his mandate expired.