A bloc of West African nations has issued a deadline to Gambia's stubborn incumbent President Yahya Jammeh, after which they have vowed to invade the country and remove him by force.

Marcel de Souza, the president of the Economic Community of West African States (ECOWAS), said members had agreed that neighbouring Senegal would lead any military intervention.

He said on Friday that Mr Jammeh has until 19 January to admit defeat and hand over the reins of power.

Mr Jammeh, who came to power in a military coup 22 years ago, lost a historic election in Gambia on 1 December. He initially said he accepted the result, before one week later declaring unspecified “irregularities” had rendered the election void.

Mr De Souza spoke to reporters in Bamako, Mali, as ECOWAS diplomats continued to push for a peaceful solution.

“The deadline is 19 January when the mandate of Jammeh ends,” Mr De Souza said. “If he doesn't go, we have a force that is already on alert, and this force will intervene to restore the will of the people.”

The overwhelming call from international leaders has been from Mr Jammeh to step aside and allow Adama Barrow, the victorious opposition coalition candidate, to become President.

But one week ago soldiers loyal to Mr Jammeh seized control of the electoral commission's offices, and the incumbent has mobilised troops across the tiny country.

The UN Security Council this week urged Gambian security forces to “demonstrate maximum restraint” and again urged Mr Jammeh to accept defeat.