By Ulf Laessing and Paul Carsten
ABUJA (Reuters) - The West African regional bloc said on Saturday it would take all necessary actions to uphold the result of a Dec. 1 election in Gambia, where veteran President Yahya Jammeh says he will not step down after losing to Adama Barrow.
ECOWAS leaders said in a communique marking the end of a summit in the Nigerian capital that they would attend the Jan. 18 inauguration of Barrow, "who must be sworn in", and guarantee the safety of the president-elect.
Barrow's surprise victory and Jammeh's initial decision to step down was seen across Africa as a moment of hope.
Jammeh announced on Dec. 9 that he would reverse that position and called for a fresh vote. That move was widely condemned, including by ECOWAS leaders who say it violates the principle of democratic accountability.
Jammeh's party is now challenging the result in Gambia's Supreme Court.
ECOWAS agreed to "respect the will of the Gambian people" and said the group had nominated Nigerian President Muhammadu Buhari to mediate.
"The authority (ECOWAS) shall undertake all necessary actions to enforce the result of the election," the group added, calling on all stakeholders inside and outside the country to show restraint.
"(ECOWAS) calls on the Gambian defense and security to perform their role in a nationalistic manner and protect lives and property," the final communique said.
A Gambian delegation led by Works Minister Bala Garba Jahumpa had arrived while the summit was already in progress to affirm Jammeh's stance, diplomats said.
ECOWAS measures could include sanctions, which could hurt Gambia because ECOWAS member Senegal is the country's only neighbor.
Jammeh's 22 years in power have been marked by allegations of human rights abuses and repression against perceived political opponents.
ECOWAS chairwoman Ellen Johnson Sirleaf went 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.
(Additional reporting by Paul Carsten; Writing by Matthew Mpoke Bigg; Editing by Catherine Evans)