West African leaders on Thursday said they have ordered the immediate deployment of a “standby force” ready to intervene in Niger, sticking with their threat of military action against coup leaders who removed the country’s president from power last month.

They provided no details of their plans, but released the announcement at the end of a crucial summit in Abuja, the capital of Nigeria, to address the crisis in neighboring Niger, where the mutinous generals who seized power more than two weeks ago have shunned mediation efforts and ignored an ultimatum to relinquish power.

“No option is taken off the table, including the use of force as a last resort,” President Bola Tinubu of Nigeria, the current chair of the West African bloc said, reading a statement at the end of the summit.

Heads of state from the Economic Community of West African States, the 15-nation regional bloc known as ECOWAS, had already threatened military intervention if Niger’s president was not reinstated by last Sunday. But heading into the summit on Thursday, their options appeared to be limited and there were doubts they would follow through on the red line they had drawn.

It was unclear how many troops would be mobilized, from which countries they would be drawn or when or where they could be deployed. Niger shares borders with four countries that are part of the bloc, but two of them, Burkina Faso and Mali, have said they would defend Niger’s junta in the event of a military intervention. Both of those countries have been suspended from the bloc because they are led by military juntas that took power in coups.

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Hopes for an end to the stalemate were further dimmed when the junta on Thursday replaced the cabinet of the ousted president, Mohamed Bazoum, with a new government made up of 21 officials led by Ali Lamine Zeine, an economist and former finance minister. The two highest-ranking officials after Mr. Zeine are both generals and coup leaders.

Omar Alieu Touray, the top administrative official of the West African bloc, said in a statement he read after the meeting, “All diplomatic efforts made by ECOWAS in resolving the crisis have been defiantly repelled by the military leadership of the Republic of Niger.”

The continuing crisis has been humbling for several powers active in West Africa, including the United States, which has bases and troops in the country but no current ambassador; France, the former colonizer, which has faced growing resentment over its presence in the region; and Nigeria, Niger’s giant neighbor to the south.

Mr. Tinubu had vowed weeks before the coup that leaders in the region would no longer tolerate unconstitutional power grabs.

Despite widespread condemnation in the West and from most West African countries, many Nigeriens have welcomed the military takeover, which they see as a welcome change from what they say was more than a decade of corruption under Mr. Bazoum and his predecessor, Mahamadou Issoufou.

In the capital, Niamey, hundreds of Nigerien civilians have stood guard at traffic circles at night, vowing to defend the generals in power against a foreign infiltration.

Until mutineers detained him on July 26, Mr. Bazoum had maintained a close security partnership with Western allies like the United States and European countries, while buying drones from Turkey and developing a pipeline project with China’s national oil company.

Whether that fragile security architecture could survive under the rule of the new generals in power, in a region plagued by frequent military takeovers and roaming Islamist insurgencies, including in Niger, is unclear.

The United States and France, two key security partners who have about 2,500 troops combined in Niger, have suspended their military assistance and called for the reinstatement of Mr. Bazoum.

But so far, mediation attempts led by Western countries and West Africa’s bloc have stalled.

Twice, the mutinous leaders have refused to meet envoys from the West African bloc. And Victoria Nuland, the acting U.S. deputy secretary of state, was denied a meeting with the junta’s leader or Mr. Bazoum when she made a surprise trip to Niger on Monday.

On Wednesday, a Nigerian religious figure, Khalifa Muhammad Sanusi, the emir of Kano, became one of the few mediators who has been allowed to meet the junta’s leader, Gen. Abdourahmane Tchiani. Details of the encounter were not made public.

Mr. Bazoum has remained stranded in his private residence with his wife and one of his sons, who is in his early 20s, for more than two weeks. The family does not have electricity or running water, and the junta has failed to provide food, according to a friend and adviser to the president who requested anonymity to discuss his situation. The friend said the family was living on reserves.

António Guterres, the secretary general of the United Nations, said on Wednesday that he was very concerned about “the deplorable living conditions” for Mr. Bazoum and his family. He called for their immediate release, along with that of several government officials who have been in custody since the mutineers removed the president from power on July 26.

The new government includes Gen. Salifou Modi, who was Niger’s military chief of staff until he was removed from his duties by Mr. Bazoum in April. An adviser to Mr. Bazoum said last week that General Modi played a central role last week in rallying the military behind the coup. Four other military officers make up the cabinet.

Many in Niamey have said they do not want Mr. Bazoum back in power. Omar Salifou, a music industry representative, stood guard at a traffic circle there on Wednesday evening, as he had on previous nights.

Amid cars honking and pushing through the crowds on the rain-soaked road, some young people could be heard shouting, in a video he sent to The New York Times, “Down with ECOWAS.”

Farnaz Fassihi contributed reporting from New York.