Gambia's President Yahya Jammeh attends the plenary session of the Africa-South America Summit on Margarita Island September 27, 2009. REUTERS/Carlos Garcia Rawlins/Files

DAKAR (Reuters) - The United Nations Special Representative for West Africa said on Wednesday that Gambia’s President Yahya Jammeh would be “strongly sanctioned” if he sought to remain in power after his mandate ends next month.

Long-ruling Jammeh lost elections to little-known businessman Adama Barrow on Dec. 1 and conceded defeat in a widely celebrated moment of democratic hope for the continent.

But he has since changed his mind and his party is now challenging the outcome at Gambia’s Supreme Court, raising the prospect that it could be overturned.

“For Mr. Jammeh, the end is here and under no circumstances can he continue to be president,” the U.N.’s Mohammed Ibn Chambas told Reuters by telephone, a day after talks between Jammeh and regional leaders failed to reach a deal to have him step down.

“The Supreme Court decision has nothing to do with the five-year mandate,” he added.

He said Jammeh would be “strongly sanctioned” if he does not step down by the end of his mandate, which ends on Jan. 18.

Asked whether military intervention was an option following the failed mediation mission, he said: “It may not be necessary. Let’s cross that bridge when we get there.”

Jammeh’s U-turn on the election outcome has prompted strong criticism.

Chambas, who has been advising the mediators, said West African heads of state would discuss Gambia further at a meeting of regional body ECOWAS in Nigeria on Saturday.