The main spokesman of the Islamic State extremist group, Abu Mohammed al-Adnani, has been killed in northern Syria, the group said Tuesday via its outlets on social media.

Al-Adnani was one of the group's senior leaders, according to the United States, which had offered a reward of up to 5 million dollars for information bringing him "to justice."

He was notorious for his audio messages calling for attacks on Islamic State's enemies, including civilian targets in the West, and he announced the group's purported establishment of a caliphate in Syria and Iraq in 2014.

"The sheikh departed while inspecting military operations in Aleppo province," Islamic State said, describing him as a "martyr" without giving any details of how he died.

Islamic State has recently lost ground to both Kurdish-led forces and Syrian rebels backed by Turkey in the Aleppo area, which was its key conduit for the entry of foreign fighters seeking to join its ranks in Syria and Iraq.

Al-Adnani, who was born Taha Sobhi Falaha in Syria according to the US, was Islamic State's best-known voice, with media appearances by its top leader Abu Bakr al-Baghdadi much rarer.

In June 2014, after the then Islamic State in Iraq and the Levant captured the city of Mosul and swathes of northern and western Iraq in a lightning offensive, al-Adnani announced that it was being renamed "the Islamic State."

Al-Baghdadi was now caliph and entitled to the allegiance of all Muslims worldwide, al-Adnani said.

As recently as May, ahead of the start of the Islamic holy month of Ramadan, he repeated his call for attacks on Western targets.

"Make this month, with God's will, a month of woes for the infidels everywhere. This [call] is especially meant for the soldiers and supporters of the Islamic State Caliphate in Europe and America," he said in an audio recording.

Islamic State's claims of atrocities such as the July truck attack that killed 84 people in Nice on the French national holiday, Bastille Day, have framed those attacks as responses to such calls.

The perpetrator of at least one Islamic State-linked attack outside the Middle East mentioned al-Adnani, in a video recorded after the attacker killed a French police officer and his companion in their home near Paris.

Al-Adnani's death comes as the Islamic State movement reels under attacks from increasingly effective Iraqi security forces and Kurdish-led forces in Syria, both backed by intensive US-led airstrikes.

The group has now lost key territory in both countries, with the Kurdish-led Syrian Democratic Forces driving Islamic State forces from much of northern Syria and the Iraqi government recapturing much of the ground lost in 2014.

According to analysis firm IHS, the Islamic State movement lost 12 per cent of its territory in the first six months of 2016 as well as suffering a drastic decline in its revenues.

The group still holds two key strongholds of al-Raqqa in Syria and Mosul in Iraq, while analysts have warned that territorial losses are likely to push the movement to intensify terrorist-style attacks such as those al-Adnani called for.

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