Gambia's President Yahya Jammeh has criticised international pressure for him to step down as he announced the appointment of a mediator to help resolve the country's political impasse.

Gambia has been thrust into a crisis following a December 1 presidential vote, which saw longtime ruler Jammeh losing to opposition leader Adama Barrow.

Jammeh initially conceded defeat but later reversed his position, lodging a legal case aimed at annulling the result and triggering new elections.

In a surprise address on national television late on Tuesday, Jammeh lashed out at "an unprecedented level of foreign interference in our elections and internal affairs and also a sustained smear campaign, propaganda and misinformation".

Leaders from the Economic Community Of West African States, or ECOWAS, bloc led by Nigerian President Muhammadu Buhari are expected to visit Gambia's capital, Banjul, on Friday with hopes of bringing a diplomatic end to the impasse.

In his TV address, Jammeh attacked ECOWAS, the UN Security Council and the African Union - all bodies that have urged him to respect the election result - for taking "unprecedented and hasty resolutions against our republic and constitution".

He also appealed for patience, asking his Gambians to "await the Supreme Court review and ruling on the election results".

On Tuesday, the Supreme Court postponed a ruling on a possible re-run of the presidential election until May, casting further doubt on whether a peaceful political transition will happen on January 19 when Barrow is scheduled to take office.

'Peaceful resolution'

Al Jazeera's Nicolas Haque, reporting from Senegal's capital, Dakar, said that Jammeh had nominated his chief justice to be a mediator in the crisis.

"He says the mediation should be with all stakeholders in order to find a peaceful resolution," Haque said.

"Jammeh is still not ready to concede defeat; he says that there will be a peaceful resolution in the days to come but gives no explanation as to how this is going to come about.

"Meanwhile, the opposition is still waiting for the inauguration, the handover of power that is supposed to happen on January 19."

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In his speech, Jammeh who has ruled Gambia for 22 years, did not budge on his refusal to acknowledge Barrow as winner of the election.

Yet, he appeared more conciliatory to his own public than in recent weeks.

"I believe we can ask Gambians to come together to resolve this and any other matter without undue external interference," said Jammeh.

The president also called for no arrests for actions relating to the pre and post-election period, without specifying what such acts would be under the law.