Abu Muhammad al-Adnani, who was the Islamic State spokesman and key strategist, in an image posted on social media in 2012. (Ho/AFP/Getty Images)

Russia claimed Wednesday that it killed one of the Islamic State’s most senior leaders with an airstrike in Syria, but U.S. officials said they had no evidence to back Moscow’s account and were examining whether the militant was hit in an American strike.

A statement from Russia’s Defense Ministry cited “intelligence channels” confirming that Tuesday’s attack near Aleppo killed Abu Muhammad al-Adnani, the Islamic State’s main spokesman and a leading strategist involved in planning attacks overseas.

Russia has been eager to show that its main role in Syria is to fight terrorism, not prop up the regime of Bashar al-Assad, and claiming responsibility for killing a senior Islamic State official would serve to bolster that case.

Russia did not provide evidence other than its statement, and the claim could not be independently verified. Earlier, the Pentagon said it had targeted Adnani in an airstrike in a different location in the Aleppo region and was still assessing that attack’s results.

Officials in Moscow have previously released statistics about Russia’s intervention that seemed exaggerated and designed to belittle the U.S. war effort. In May, Evgeny Lukyanov, deputy head of Russia’s Security Council, estimated that Russian forces had killed 28,000 radical Islamists in Syria using several dozen warplanes in an intervention that began in September. U.S.-led airstrikes in the country had killed just 5,000 fighters over two years, Lukyanov said.

The same month, a spokesman for the Russian Defense Ministry told The Washington Post that he estimated civilian casualties from months of Russian airstrikes to be zero because Russian warplanes never targeted population centers.

Pentagon spokesman Peter Cook told reporters that U.S. officials had “no information” to support Russia’s claim that it killed Adnani. White House press secretary Josh Earnest also said he was “not aware of any facts” to back Moscow’s report.

The U.S. strike was conducted by an unmanned drone firing Hellfire missiles on the northern edge of the town of Bab, about three miles from the town center, a U.S. official said. The Pentagon believes that Adnani was traveling in a vehicle at the time with at least one other person, said the official, who spoke on the condition of anonymity to discuss details of the attack.

The Russian statement said the airstrike by a Syrian-based Su-34 bomber killed as many as 40 Islamic State fighters, including Adnani.

According to the Russian Defense Ministry, the strike took place near Maarat Umm Hawsh, a village north of Aleppo and about 16 miles west of Bab, which is controlled by the Islamic State.

The United States and Russia have both intervened in the Syrian conflict with airstrikes and other military support, but they remain fundamentally opposed over the fate of Assad’s regime.

Russia, which backs Assad, has portrayed its intervention primarily as a crusade against the Islamic State and other groups it deems “terrorists.” The West has accused Russia of indiscriminate bombing of civilians and more-moderate rebel groups, including those backed by the United States.

If a Russian attack did kill Adnani, it would be an unusual, targeted strike against a senior Islamic State official by Moscow, exhibiting Russia’s growing capabilities in Syria and bolstering the Kremlin’s information campaign that its intervention is legitimate.

The Islamic State issued a statement announcing Adnani’s death but gave no information on who carried out the attack.

“Al-Adnani’s removal from the battlefield would mark another significant blow to ISIS,” Cook said, using an acronym for the Islamic State.

“He has coordinated the movement of ISIS fighters, directly encouraged lone-wolf attacks on civilians and members of the military, and actively recruited new ISIS members,” Cook said Tuesday.

The Islamic State urged followers to avenge Adnani’s death and vowed to keep fighting even amid setbacks on the battlefield.

In an online edition of the Islamic State’s newspaper al-Naba, distributed hours after the announcement of Adnani’s death, the group told its fighters to persevere.

“This religion will always stand, unharmed by the death of any person,” the news site said. Supporters should “stand up and die” just as Adnani did, al-Naba said, according to a translation carried by the SITE Intelligence Group, which monitors militant statements.

The reported death of Adnani coincides with a string of setbacks for the militants in Iraq and Syria, where they have been rapidly losing control of some of their most significant strongholds.

Most recently, the group lost Jarabulus, a transit point for foreign fighters on the Turkish border that was recaptured by a joint force made up of Syrian rebels and Turkish troops.

Most of the Islamic State fighters who had been based in Jarabulus fled ahead of the advancing force to Bab, a strategically important town about 30 miles east of the city of Aleppo that is expected to become the venue for one of the next important battles.

A Syrian news website, Syrian View, reported that Adnani was killed just outside Bab around 3 p.m. by a coalition airstrike that hit his car, killing him and another Islamic State fighter.

He had been headed to the front lines near the nearby town of Manbij, which was captured two weeks ago by a U.S.-backed Kurdish-led force, the website said. The report could not be independently confirmed.

Meanwhile, Islamic State supporters took to various forms of social media to lament the news and call for revenge attacks.

“The Muslims are revived by the blood of those who you kill, and the fire of the jihad is ignited with it, and its flames intensify,” said the Nashir Foundation, a pro-Islamic State group, on its Telegram channel, according to the SITE Intelligence Group.

“Today is the day of revenge. Kalashnikovs are not enough. Today is the Duma [bomb] day,” said another posting by a purported fighter using the name Abu Asim al-Masri. Dugma is a local colloquialism used to refer to the truck-size suicide bombs that are a hallmark of the Islamic State.

Adnani had been widely tipped as a likely successor to the current head of the Islamic State, Abu Bakr al-Baghdadi, and the language used in the eulogies further indicated the esteem in which he was held by the group.

“The way he’s being described in the obituaries, the indications of lineage, the epithets, point to him having been the next caliph,” said Aymenn al-Tamimi, an analyst specializing in militant groups with the Philadelphia-based Middle East Forum.

Adnani was the most recognizable figure in the group after Baghdadi, especially now that dozens of its senior leaders have been killed.

“I can’t think of many figures who are still alive,” Tamimi said. “This is part of a wider decline of the Islamic State; the territorial losses, manpower losses, loss of senior personnel.”

Sly reported from Beirut and Gibbons-Neff from Washington. Zakaria Zakaria in Gaziantep, Turkey, Loveday Morris in Baghdad and Missy Ryan in Washington contributed to this report.