A Russian air strike killed the Islamic State group's spokesman and chief strategist, says Russia's defence ministry.
The ministry says its intelligence confirmed that a Russian air strike on Aleppo province killed 40 IS militants, including spokesman Abu Muhammed al-Adnani.
His death is a major blow to the extremist group, which has been retreating in Syria and Iraq.
The United States said it launched an air strike targeting al-Adnani in the Syrian city of al-Bab, which is north-east of Aleppo.
Pentagon press secretary Peter Cook said on Tuesday the US was "still assessing the results of the strike."
Al-Adnani persistently called for attacks against the West.
The IS group said its spokesman and chief strategist, who laid out the blueprint for the group's attacks against the West, was killed overseeing operations in northern Syria.
Abu Muhammed al-Adnani was "martyred while surveying the operations to repel the military campaigns in Aleppo," said the IS-run Aamaq news agency
A later statement issued by the group in Aleppo province vowed to avenge his death.
Al-Adnani, whose real name is Taha Sobhi Falaha, persistently called for attacks against the West.
These resulted in 130 people killed and hundreds more injured in the November 13 coordinated attacks in Paris that hit a concert hall, a stadium and restaurants and bars.
He was a Syrian who was born in the northern province of Idlib and is believed to have been in his late 30s.
He crossed the border and joined al Qaida in Iraq, a precursor to IS, after the 2003 US-led invasion.
He formally declared the establishment of a caliphate, or Islamic state, in late June 2014.
This stretched across parts of Syria and Iraq, under the leadership of Abu Bakr al-Baghdadi, and al-Adnani demanded allegiance from Muslims worldwide.
The powerful orator became the voice of IS.
He delivered fiery sermons urging followers to kill civilians in nations supporting the US-led coalition against the group in audio files released online.
He also called for massive attacks during Ramadan earlier this year
This call resulted in the bloodiest Muslim holy month in recent memory.
Followers of IS carried out attacks on several continents, including the Orlando shooting, the Nice truck attack in France and a massive suicide bombing in downtown Baghdad.
Aleppo is a current focal point of the civil war in Syria, where IS, Syrian Kurdish forces, Turkey-backed rebels and president Bashar Assad's forces are vying for control.
The province is frequently struck by both US-led coalition air strikes and Russian air raids.
IS has suffered a string of defeats in recent weeks, including in Aleppo province, where Turkish troops and allied Syrian rebels drove the group out of the border town of Jarablus last week.
The group has lost its strongholds in Fallujah and Ramadi, in the western Anbar province of Iraq.
It still controls Mosul, but Iraqi forces are gearing up for a long-awaited operation to retake the country's second largest city.
It has also lost some of its most senior commanders and founding members over the past year.
These included its "minister of war" Omar al-Shishani, feared Iraqi militant Shaker Wuhayeb, also known as Abu Wahib, and a top finance official known by several names, including Abu Ali Al-Anbari.