President Putin’s spokesman, Dmitry Peskov, on Friday rejected allegations that the Kremlin was behind a plane crash that is presumed to have killed mercenary leader Yevgeny Prigozhin, whose brutal fighters were feared in Ukraine, Africa and Syria and conducted a brief but shocking mutiny in Russia two months ago.

Prigozhin, who was listed among those on board the plane, was eulogized by Mr. Putin, even as suspicions grew that the Russian leader was behind a Wednesday crash that many saw as an assassination.

A preliminary American intelligence assessment concluded that the plane was downed by an intentional explosion. One of the American and Western officials who described the initial assessment said it determined that Prigozhin was “very likely” targeted and that the explosion falls in line with Mr. Putin’s “long history of trying to silence his critics.”

The officials, who spoke on the condition of anonymity because they were not authorized to comment, did not offer any details about what caused the explosion, which was widely believed to be vengeance for the mutiny in June that posed the biggest challenge to the Russian leader’s 23-year rule.

“Right now, of course, there are lots of speculations around this plane crash and the tragic deaths of the passengers of the plane, including Yevgeny Prigozhin,” Mr. Peskov told reporters during a conference call. “Of course, in the West those speculations are put out under a certain angle, and all of it is a complete lie.”

Asked by the Associated Press whether the Kremlin has received an official confirmation of Prigozhin’s death, Mr. Peskov referenced Mr. Putin’s remarks from a day earlier: “He said that right now all the necessary forensic analyses, including genetic testing, will be carried out. Once some kind of official conclusions are ready to be released, they will be released.”

Britain’s defense ministry said Friday that Prigozhin’s presumed death could destabilize the Wagner Group.

“The demise of Prigozhin almost certainly would have a deeply destabilizing effect on the Wagner Group. His personal attributes of hyperactivity, exceptional audacity, a drive for results and extreme brutality permeated Wagner and are unlikely to be matched by any successor,” the ministry wrote in a statement.

Wagner mercenaries were key elements of Russia’s forces in its war in Ukraine, particularly in the monthslong fight to take the city of Bakhmut, the conflict’s most grueling battle. Wagner fighters also have played a central role projecting Russian influence in global trouble spots, first in Africa and then in Syria.

“When it comes to the future (of Wagner), I can’t tell you anything — I don’t know,” Mr. Peskov said.

The jet crashed Wednesday soon after taking off from Moscow, carrying Prigozhin, six other Wagner members, and a crew of three, according to Russia’s civil aviation authority. Rescuers found 10 bodies, and Russian media cited anonymous sources in Wagner who said Prigozhin was dead. But there has been no official confirmation.

President Biden, speaking to reporters Wednesday, said he believed Mr. Putin was likely behind the crash.

“I don’t know for a fact what happened, but I’m not surprised,” Mr. Biden said. “There’s not much that happens in Russia that Putin’s not behind.”

The passenger manifest also included Prigozhin’s second-in-command, whose nom de guerre became the group’s name, as well as Wagner’s logistics chief and at least one possible bodyguard.

It was not clear why several high-ranking members of Wagner, including top leaders who are normally exceedingly careful about their security, would have been on the same flight. The purpose of their trip to St. Petersburg was unknown.

Russian authorities have said the cause of the crash is under investigation.

In his first comments on the crash, Mr. Putin said the passengers had “made a significant contribution” to the fighting in Ukraine.

“We remember this, we know, and we will not forget,” he said in a televised interview with the Russian-installed leader of Ukraine’s partially occupied Donetsk region, Denis Pushilin.

Mr. Putin recalled that he had known Prigozhin since the early 1990s and described him as “a man of difficult fate” who had “made serious mistakes in life, and he achieved the results he needed — both for himself and, when I asked him about it, for the common cause, as in these last months. He was a talented man, a talented businessman.”

Russian state media have not covered the crash extensively, instead focusing on Putin’s Thursday remarks to the BRICS summit in Johannesburg via video link and Russia’s invasion of Ukraine.

Sergei Mironov, the leader of the pro-Kremlin Fair Russia party and former chairman of the upper house of the Russian parliament, said on his Telegram channel that Prigozhin had “messed with too many people in Russia, Ukraine and the West.”

“It now seems that at some point, his number of enemies reached a critical point,” Mironov wrote.

Numerous opponents and critics of Mr. Putin have been killed or gravely sickened in apparent assassination attempts, and American and other Western officials long expected the Russian leader to go after Prigozhin, despite promising to drop charges in a deal that ended the abortive June mutiny.

Mr. Putin first denounced the rebellion as “treason” and a “stab in the back,” but soon made a deal that saw an end to the mutiny in exchange for an amnesty for Prigozhin and his mercenaries and permission for them to move to Belarus.

Now, many are suggesting the punishment has finally come.