NIAMEY Niger mediation efforts appeared stalled on Wednesday after the leaders of a July coup rejected another diplomatic mission, and neighbouring allies who back the armed takeover appealed to the United Nations to prevent a military intervention.

Niger's junta denied entry to a joint delegation from African countries and the United Nations on Tuesday, resisting pressure to negotiate ahead of a summit on Thursday in which the main regional bloc, the Economic Community of West African States (ECOWAS), will discuss possible use of force.

The move has dimmed hopes of a diplomatic resolution to the standoff that threatens to further destabilise West Africa's Sahel region - one of the world's poorest that is already dealing with a string of coups and a deadly Islamist insurgency.

Neighbouring Mali and Burkina Faso, also ruled by military juntas, asked the U.N. Security Council in a letter on Tuesday to prevent any armed action against Niger, saying it would have "unpredictable" consequences and lead to the break-up of ECOWAS.

"The Transitional Governments of Burkina Faso and the Republic of Mali appeal to the primary responsibility of the Security Council... to use all means at its disposal to prevent armed action against a sovereign state," said the letter signed by both countries' foreign ministers and posted on X by the Malian foreign ministry.

They said they were committed to finding solutions through diplomacy and negotiation, but did not give details.

Mali and Burkina Faso had previously vowed to come to Niger's defence if the regional bloc intervened, saying they would consider that a declaration of war against them.

U.S. Secretary of State Antony Blinken said late on Tuesday that he had spoken to ousted Niger President Mohamed Bazoum to express continued efforts to find a peaceful resolution to the crisis.

"The United States reiterates our call for the immediate release of him and his family," he posted on the social media platform X, formerly known as Twitter. Bazoum has been detained in his residence since the July 26 coup.

Nigeria's President and ECOWAS chairman Bola Tinubu imposed more sanctions on Tuesday aimed at squeezing entities and individuals involved in the takeover, and said all options were still on the table.

ECOWAS has said that the use of force would be a last resort if the soldiers do not step down and free Bazoum. The bloc's defence chiefs have agreed on a possible military action plan, which heads of state are expected to consider at their summit in the Nigerian capital, Abuja.

(Writing by Nellie Peyton; Editing by William Maclean)