Several Kurdish sources and Kurdish Defence Units (YPG) claimed one on the most influential and known leaders of the Islamic State of Iraq and Levant (ISIS), Chechen national Abu Omar al-Chechen (alternatively Abu Omar al-Shishani), has been killed in fight.

The YPG have launched an offensive in the village Yusif near their headquarters in Celaxa (in the city Girke Blank in the province of al-Hassaké) against rebels of the al-Nusra front.

In the predominantly Kurdish areas in Aleppo YPG is fighting against rebels of the Free Syrian Army (FSA). At least four rebels were killed on Friday.

The YPG also reported that in the battle in which commander Abu Omar al-Chechen died another 15 militants have been killed. Clashes took place in Atimah, Idlib.

Eighteen al-Nusra and ISIS rebels were reported killed by the YPG in ongoing fighting in the vicinity of Efrin and in Aleppo.

On social media a twitter war erupted between supporters of the YPG one hand and supporters of Islamist rebels on the other. Islamists deny the death of al-Chechen, although YPG is said to have claimed having the body of the islamic commander.

WHO IS AL-CHECHEN

Abu Omar al-Chechen was a commander from the Russian Caucasus. He has been a key leader in the Muhajireen Brigade (also Jaish al-Muhajireen wal-Ansar, Army of Emigrants and Helpers), a jihadist group part of the Islamic State of Iraq and Levant fighting against the regime of President Bashir al Assad. The Muhajireen Brigade, whose members include fighters from the Islamic Caucasus Emirate, has played a vital role in overrunning several major Syrian military installations over the past year.

Abu Omar "is an Emir of Mujahideen Brigade of Muhajirs, or migrants which also includes volunteers from the Caucasus Emirate," according to Kavkaz Center, a media arm of the al Qaida-linked Islamic Caucasus Emirate. Kavkaz Center posted a video of an appeal by Abu Omar on its English-language website on 7 February 2013.

In the video, Abu Omar, sporting a red beard and a cap, is seen seated among 19 heavily armed jihadists, many of whom are masked. Two of the jihadists are seen holding al-Qaida in Iraq's flag.

"Chechen fighters," often described as fighters from the Caucasus and southern Russia, have been spotted on the Syrian battlefield for months.

The Muhajireen Group is known to have participated in two other major assaults against Syrian military bases since the October 2012 operation in Aleppo.

In mid-December 2012, the Muhajireen Group teamed up with the Al Nusrah Front to overrun the Sheikh Suleiman base, or Base 111. Arab and Central Asian fighters are reported to have participated in the battle.