ABUJA (Reuters) – West Africa’s regional bloc ECOWAS must ensure Gambia’s President Yahya Jammeh steps down when his mandate ends in January because he lost an election, the bloc’s chairwoman, President Ellen Johnson Sirleaf of Liberia, said on Saturday.

Her comments came at the start of an ECOWAS summit in the Nigerian capital aimed at deciding on action against Jammeh in the face of what regional leaders say is a challenge to its principles of democratic accountability.

Johnson Sirleaf did not spell out what measures the bloc would take but they could include sanctions, which could hurt Gambia because ECOWAS member Senegal is the country’s only neighbour.

“It’s now important that the authority at this summit considers measures to bring the matter to a successful conclusion before January 19 … when the mandate of the current President expires,” Johnson Sirleaf told the summit.

Jammeh’s 22 years in power have been marked by allegations of human rights abuses and repression against perceived political opponents. He lost a Dec. 1 election to little-known challenger Adama Barrow and is due to step down on Jan. 18.

Initially, he conceded defeat but a week ago he rejected the results and called for a fresh vote in a move that was widely condemned. His party is now challenging the result in Gambia’s Supreme Court.

Johnson Sirleaf’s remarks follow a mission to Gambia this week accompanied by the leaders of several West African countries including Nigeria and Ghana, whose President John Mahama lost a Dec. 7 election and said he would step down.

(Reporting by Ulf Laessing and Paul Carsten; writing by Matthew Mpoke Bigg; editing by Jason Neely)