In a statement, the Defense Ministry said that the Russian Air Force struck a meeting of Islamic State leaders on May 28 outside Raqqa, Syria, the group’s de facto capital, possibly killing Mr. Baghdadi.
The statement offered no explanation for the two-week delay in publicizing the airstrike. And it was also not clear whether the Russian military had known in advance that Mr. Baghdadi was at the gathering, or had learned of this possibility only after the strike was carried out.
Rumors of Mr. Baghdadi’s death had circulated for months before the Russian announcement. But a Pentagon spokesman, Capt. Jeff Davis of the Navy, said on Friday: “We have no information to corroborate those reports.”
A senior Defense Department official confirmed Friday that Russian warplanes were “very active” carrying out airstrikes in the area south of Raqqa where Russian officials say Mr. Baghdadi may have been killed.
But the official, echoing public statements from the Pentagon, said he could not corroborate Russian reports of Mr. Baghdadi’s death.
Col. Ryan S. Dillon, a spokesman for the United States-led coalition fighting the Islamic State in Syria and Iraq, said that American air commanders in Qatar speak almost daily on a special hotline with their Russian counterparts, primarily to avoid any midair accidents by warplanes flying missions in Syria. He said in an interview that analysts were now going back over the reports from May 28 and the subsequent days to see what the Russians had said about flight operations.
Nothing has been heard from Mr. Baghdadi publicly since November, when the Islamic State released a blistering audio recording in which he urged forces to remain firm in the face of the American-backed Iraqi offensive in Mosul.
Rami Abdul Rahman, the founder of the Syrian Observatory for Human Rights, which closely monitors military operations in Syria, said it had no record of senior Islamic State leaders being in the area around Raqqa at the time of the strike. “It’s illogical for ISIS senior leaders to stay in Raqqa amid this military operation,” he said, adding that senior leaders had already decamped southeast, to the area around the city of Deir al-Zour, closer to the border with Iraq.
Laith Alkhouri, a director at Flashpoint, a business risk intelligence company in New York that tracks militant threats and cyberthreats, also expressed skepticism.
“At this time, I’m not seeing credible chatter to verify the claim by Russia,” he said. “Al-Baghdadi has been claimed killed multiple times before, and none of the previous claims proved legitimate.”
Russia’s foreign minister, Sergey V. Lavrov, himself introduced a note of caution when asked about the Russian military’s reports. “So far, I do not have 100 percent confirmation of this information,” he said at a news conference.
Mr. Lavrov also cautioned that the Islamic State was likely to survive the possible death of its leader. “Examples of such actions to destroy or ‘behead’ a terrorist group have always been presented with great enthusiasm,” he said. “However, history shows that the fighting capacity of these structures was restored.”
Russia intervened in the Syrian civil war in 2015; it said at first that cargo planes flying to a Syrian air base carried only humanitarian aid, but later openly announced a military operation. The Kremlin’s stated goal was fighting the Islamic State, lest it gain a stronghold in Syria not far from restive, predominantly Muslim regions in southern Russia.
But the Obama administration said that the pattern of airstrikes showed that Russia’s real intention was to prop up the government of President Bashar al-Assad, a Russian ally battling a range of opposition groups, including moderate rebels. The killing of the Islamic State’s leader, if confirmed, would help bolster Russia’s initial justification for its intervention — that its goal all along was to fight terrorism.
In its statement on Friday, the Russian Defense Ministry said that it had “warned the American side about the strike in advance,” but it was unclear whether Russia had shared intelligence on the meeting or cooperated in any other way with the Western-led coalition fighting in Syria. The United States and Russia do not typically share intelligence from their missions in Syria.
The Russian military does not draw a sharp separation between its psychological warfare operations and its media office, meaning the statement’s purpose could be tactical and intended to assist Russian forces in Syria. A claim that the terrorist leader had been killed, regardless of the evidence, would sow doubt among Islamic State fighters.
Russia is believed to have an extensive intelligence operation targeting the Islamic State that makes use of the large numbers of Muslims from former Soviet states who have joined the group. A former Islamic State military commander, for example, was an ethnic Chechen from Georgia.
Whether this window into the Islamic State’s activities helped the Russian military to choose a target was not clear.
The Russian military said two models of Sukhoi fighter jets, Su-34s and Su-35s, had carried out the strike, on what it described as “high level commanders of the terrorist group within the so-called military council of the Islamic State,” Interfax reported.
The strike also killed 30 field commanders and as many as 300 fighters, the military said. The strike lasted from 12:35 a.m. until 12:45 a.m., according to Interfax.
Russia learned of the meeting late in May, and while verifying the information ascertained that the commanders intended to discuss the retreat of their fighters from Raqqa, the Defense Ministry said.
The extent to which Mr. Baghdadi exerted day-to-day control over the Islamic State’s activities is not fully clear, but his death would be a major blow, easily the most prominent since Osama bin Laden, the leader of Al Qaeda, was killed in an American operation in Pakistan in 2011.
Mr. Baghdadi, who is in his 40s, emerged from the jumble of Sunni extremist elements that battled the American forces and Iraq’s new Shiite-led government in the decade after the 2003 invasion that topped Saddam Hussein. He was a follower of the Jordanian militant Abu Musab al-Zarqawi, who formed a branch of Al Qaeda in Iraq, but eventually fell out with Al Qaeda because his wanton killings of Shiites were too brutal even by Qaeda standards.
Mr. Baghdadi was named the head of the Islamic State in 2010. After the Arab Spring uprisings of 2011, Syria became a fertile ground for jihadists like Mr. Baghdadi, who exploited the power vacuum left by the violent challenge to Mr. Assad.
Using porous borders between Syria and Iraq, the new jihadists overpowered Shiite-led authorities and rival Sunni factions in both countries, and established a stronghold in an overwhelmingly Sunni area. In the summer of 2014, the group declared itself a caliphate, a successor to early leaders of Islam. It has displayed a sophisticated command of social media to recruit potential jihadists from around the world, and to sow terror in the West.
One of Mr. Baghdadi’s closest associates — Abu Muhammad al-Adnani, who headed the group’s efforts to build a global terrorism network, including overseeing attacks in Paris and Brussels — was killed in an airstrike in August. The governments of Iran, Iraq, Russia and the United States have pounded Islamic State strongholds, and Raqqa has been under attack in recent weeks.