West Africa's regional bloc ECOWAS must ensure The Gambia's President Yahya Jammeh steps down because he lost an election, the bloc's chairwoman has said.

President Ellen Johnson Sirleaf of Liberia, the chair of the Economic Community of West African States, urged the leaders at the bloc's summit in the Nigerian capital of Abuja on Saturday to decide "measures to bring this matter to successful conclusion before January 19", when Jammeh's mandate expires.

UN tells Gambia's Yahya Jammeh to step down

She did not elaborate on what measures the bloc would take, but they could include sanctions.

Jammeh's 22 years in power have been marked by allegations of human rights abuses and repression against perceived political opponents.

He lost a December 1 election to little-known challenger Adama Barrow.

Initially, he conceded defeat but rejected the results later, calling for a fresh vote. 

Gambian security forces seized the country's Independent Electoral Commission earlier this week. His party is now challenging the result in The Gambia's Supreme Court.

On Friday, Marcel de Souza, the bloc's president, said a military intervention must be considered if diplomacy failed to persuade Jammeh to step down.

READ MORE: Gambia's Jammeh rejects result of presidential election

Ebrima Sall, a Gambian senior social scientist, told Al Jazeera the business community, religious leaders and bar association have been calling on Jammeh to step down.

"Everybody is now speaking out," said Sall. "They have overcome the fear that was there and they are all calling upon him to leave and accept the verdict. They know the majority wants him to leave. Otherwise, he will be even more isolated internally, let alone internationally."

The United Nations, the United States and leading West African countries have condemned him.

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Source: Al Jazeera and news agencies