The Kremlin on Wednesday acknowledged for the first time that a plane carrying Yevgeny Prigozhin, mercenary chief of the paramilitary Wagner Group, was possibly downed on purpose.

Kremlin spokesman Dmitry Peskov told reporters that investigators are looking into the possibility that the 62-year-old Prigozhin was assassinated.

"It is obvious that different versions are being considered, including the version -- you know what we are talking about -- let's say, a deliberate atrocity," Peskov said when asked about the investigation.

The plane carrying Prigozhin and nine others crashed in Russia's Tver region on Aug. 23, killing everyone aboard, according to the press service of Russia's Federal Air Transport Agency.

The crash came about a month after Prigozhin led a chaotic armed rebellion that threatened the longstanding leadership of Russian President Vladimir Putin.

On Friday, Peskov denied speculation that the Kremlin was involved in the plane crash, calling the allegation an "absolute lie."

The crash investigation is being conducted by the Russian Investigative Committee as a domestic incident and Peskov said that allowing in international investigators "is out of the question."

-ABC News' Will Gretsky

Ukraine conducted a massive drone attack in Crimea Thursday night into Friday morning, Ukraine Defense Intelligence spokesman Andrii Yusov told ABC News.

An attack was made on the Russian 126th Separate Guards Coastal Defense Brigade of the Black Sea Fleet in the village of Perevalne, Yusov said.

"We are still calculating enemy losses at the moment," Yusov said

-ABC News' Ellie Kaufman

The explosion that downed a plane carrying Wagner Group leader Yevgeny Prigozhin and nine others in Russia was likely caused by a bomb, two U.S. officials told ABC News on Friday.

A senior U.S. official said the preliminary belief is that the private jet was downed Wednesday by an explosion on board, potentially caused by a well-placed bomb.

Another U.S official said the United States believes that a bomb was very likely the cause of the explosion.

-ABC News' Josh Margolin and Luis Martinez

Russian President Vladimir Putin had no recent meetings with Yevgeny Prigozhin, Kremlin spokesman Dmitry Peskov said Friday.

We don't know yet how long the investigation procedures will be," Peskov said when asked if Putin will attend Prigozhin's funeral.

"The president has a busy schedule in general," he added.

Peskov would not comment on the cause of the plane crash.

-ABC News' Anastasia Bagaeva and Ellie Kaufman

The United States "is prepared to defend every inch of NATO territory," National Security Advisor Jake Sullivan said Wednesday, amid Russia's threats of military retaliation if Sweden and Ukraine officially join the alliance.

"At the end of the day, Russia does not get a vote in who joins NATO," Sullivan told ABC News' George Stephanopoulos during an interview Wednesday on "Good Morning America."

"If Russia does choose to test Article 5, President Biden has said he is prepared to defend every inch of NATO territory," he added. "And later tonight, when he addresses a large crowd in Vilnius in a speech about his vision for Euro-Atlantic security and for American leadership, he will reinforce this point about the sanctity of Article 5."

Article 5 of the NATO treaty states that the signed parties "agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all."

Sweden and Finland, which have historically embraced neutrality, applied for NATO membership together last year despite warnings from Moscow against doing so, as Russian forces invaded Ukraine. Finland officially joined the alliance in April following Turkish ratification, but Turkey and Hungary have yet to ratify Sweden’s membership. NATO Secretary-General Jens Stoltenberg announced Monday that Turkish President Recep Tayyip Erdogan has agreed to send Sweden’s NATO accession documents to Turkey’s Parliament for approval "as soon as possible," one of the last steps in the membership process.

Meanwhile, Ukraine is pushing hard for NATO membership as Russia’s war presses on. Stoltenberg announced Tuesday that the bloc "will issue an invitation for Ukraine to join NATO when allies agree and conditions are met," pledging to speed up the process by eliminating bureaucratic hurdles, but without offering a timeline.

"All 31 allies got together yesterday to say with one voice that Ukraine's future is in NATO. The real question is just how we work down the pathway to get them in," Sullivan told ABC News on Wednesday. "So the signal that NATO sent yesterday was very positive to Ukraine about its future prospects for joining NATO, and we will work in terms of the democratic and security sector reforms that are necessary. And in the meantime, we're not just going to sit around. We're going to provide Ukraine with the weapons and military assistance it needs to defend its territory against Russia and to deter future aggression from Russia."

Ukraine has requested F-16 fighter jets and long-range missiles to aid its counteroffensive against Russian forces. Sullivan said the U.S. has already taken steps with NATO allies to begin the training of Ukrainian pilots on F-16s, which "will take some time," and then the aircraft will be transferred to Ukraine "likely from European countries that have excess F-16 supplies." As for the long-range missiles, Sullivan said the U.S. will continue to look into the issue and discuss it with Ukraine.