Military juntas in Mali, Niger and Burkina Faso, according to a senior U.S. military official, are now experiencing “buyer’s remorse” after ousting U.S. and French troops over the past three years and, to varying degrees, accepting Moscow’s help fighting al Qaeda and Islamic State insurgents.

“They’re starting to ask for help—the Malians in particular,” said the senior U.S. military official, adding that any help would most likely come in the form of training.

Pentagon strategists, constrained by American laws and policies limiting security assistance to military governments, hope to sideline the Russians and get back into the West African security business. One plan under U.S. consideration is to have third countries—especially Morocco—train local armies to fight extremists.

At the same time, Erik Prince, the well-connected American defense contractor, is in talks about providing security services to African governments, according to people familiar with the approach.

The shift shows how Russia’s influence across the Sahel, the semidesert band stretching across Mali, Burkina Faso, Niger and beyond, is receding after the years when Prigozhin was one of the most powerful figures there. Security experts and Western defense officials now say Moscow’s involvement might instead have contributed to a worsening security outlook in the region, so much so that the Sahel is perhaps the hottest battlefield in the global contest between Islamist militants and the West and its allies.

Over the past year, almost 11,000 people have died in connection to the Islamist insurgency in the Sahel, according to data analyzed by the Pentagon-funded Africa Center for Strategic Studies, around half of whom were killed in direct fighting.

The uprising began in Mali more than a decade ago, but the bulk of attacks now take place in Burkina Faso, with insurgents extending their ambitions south toward coastal states along the Atlantic’s Gulf of Guinea.

The Russian government and Defense Ministry didn’t respond to requests for comment. Nor did spokespeople for the Africa Corps or Wagner Group.

Moscow’s struggles in Africa show the limits of its power, especially when its best military units are fighting in Ukraine, according to European security officials.

A thousand Wagner mercenaries first arrived in Mali in late 2021, with the nation’s government paying $10 million a month for help fighting the insurgents. It was a battle French and United Nations troops, as well as American advisers, had been unable to win despite years of effort.

The Wagner mercenary mission had already turned into a fiasco by the time Africa Corps reinforcements arrived, according to an investigation released in August by the Sentry, a group co-founded by actor George Clooney that advocates against corruption and illicit financial flows.

Some of Wagner’s troubles were self-inflicted. In Mali, the mercenaries’ uncoordinated, brutal raids on civilian settlements “created chaos and fear within the Malian military hierarchy,” deterring informers from collaborating and creating recruitment opportunities for jihadists, the Sentry’s report said.

The Wagner mercenaries had hoped to cash in on their African ventures. But they found themselves unable even to reach a giant Malian gold deposit they were planning to mine because the area wasn’t safe enough to visit, the report said. “Wagner has seemingly gone unpaid for months and failed to obtain access to lucrative natural resources,” the Sentry analysts concluded. The report said its deployment in Mali wasn’t “a worthwhile investment for any party involved.”

In June, Wagner mercenaries left Mali, their reputation in tatters because of their failure to blunt insurgent advances and their history of slaughtering civilians in the name of providing security.

“Wagner’s failures in Mali should serve as a warning to other regimes across Africa that Russia is neither a reliable partner nor a quick fix for your problems,” said Justyna Gudzowska, Sentry’s executive director.

The Kremlin sent its Africa Corps, which is directly affiliated with its Ministry of Defense, to mend ties with authorities in Bamako, the Malian capital. “Russia is not losing ground,” Africa Corps said in a social-media post in June. “On the contrary, it continues to support Bamako at a more fundamental level.”

Just over a week later, however, a convoy of Africa Corps and Malian fighters drove into an ambush in the country’s Saharan north. Tuareg rebels, who sometimes fight alongside Islamist militants, destroyed half of the 40 armored vehicles in the convoy and killed dozens of fighters, according to European officials.

Russia is also facing new obstacles elsewhere on the continent.

In the Central African Republic, where the government is trying to put down a persistent insurgency, Wagner mercenaries had a hand in a variety of business ventures, from mining gold to providing presidential bodyguards. Moscow is pushing President Faustin-Archange Touadéra to replace the existing deal he cut with the mercenaries with a cash contract with the Africa Corps, say European officials.

The new Russian contractors mostly remain in their barracks to focus on training the local army, the European officials say, limiting their effectiveness.

Russian attempts to secure a deep foothold in Sudan’s civil war have failed. Under Prigozhin, Wagner had extracted gold in partnership with Mohamed Hamdan Dagalo, the rebel leader the U.S. has accused of genocide. After Prigozhin’s death, Moscow dispatched new personnel to guard mines and man the antiaircraft weapons that protected them, according to local activists and residents.

The effort flopped. Gunmen have conducted holdups around the mines, blocking fuel supplies and preventing local workers from reaching the mines, these people say. Sudanese government warplanes have repeatedly struck the mines, and the Russian guards left in May, activists and residents say.

A spokesman for Dagalo didn’t return a request for comment.

Similar Russian plans to develop a significant presence in new parts of Africa have also been rebuffed. In Burkina Faso, in the Sahel, a 300-strong Russian mercenary force from a company called Bear arrived in May 2024, but was recalled three months later to fight in Ukraine.

A small Africa Corps contingent is in the country to train the military in drone operations and protect junta leader Ibrahim Traoré, according to European security officials. But the ruling junta there has made it clear it wants Russia’s presence to remain limited, the European officials said.

The Burkina Faso military didn’t return a request for comment.

Moscow’s challenges create a fresh opening for Western powers to regain the access and influence in the Sahel they had lost after the military coups. In Niger, where a coup toppled a pro-Western president in 2023, Russian military instructors initially deployed to an air base last year, as French and U.S. counterterrorism forces were forced to leave.

Prince, an ally of President Trump, recently visited the country’s capital, Niamey, and later offered counterterrorism services, according to people familiar with the outreach.

In July, Rudolph Atallah, a Trump administration counterterrorism adviser, visited Mali to offer American assistance. He didn’t return a request for comment.

That same month, the commander of the French army command in Africa, Maj. Gen. Pascal Ianni, visited the Central African Republic to discuss the resumption of military training. Since then, members of the country’s security forces have been sent for training in France, according to the French Embassy there.

Write to Benoit Faucon at benoit.faucon@wsj.com and Nicholas Bariyo at nicholas.bariyo@wsj.com