Kurdish and rebel armed groups fighting extremist militants the Islamic State (IS) in Syria have agreed to join forces against their common enemy, the latest in a complex and shifting series of battlefield allegiances.

Units affiliated with the Kurdish People's Protection Units (YPG), Islamic Front (IF) and US-backed Free Syrian Army (FSA) will form a new coalition named Burkan al-Forat (Euphrates Volcano) after the river which flows through the region in which the agreement was formalized.

The alliance was announced via a video posted on Wednesday in which a man read a statement in front of an armed group including a number of women fighters backed by YPG pennants alongside the three-starred flag of Syrian opposition groups and others. The UK-based Syrian Observatory for Human Rights also reported the news.

"Due to the behaviour and actions of IS gangs; of injustice and corruption in the country, humiliation of people, and their seeking to suppress the blessed Syrian Revolution we — the brigades and battalions of East al-Tawhid, Swar al-Raqqa, Shams al-Shamal, YPG, Women Protection Units (YPJ), Saraya Jarabulus, Kurdish Front, Umana al-Raqqa brigade, Qasas army and al-Jihad Fi Sabil Allah Brigade — have declared a joint coordination military room called Burkan al-Forat to stand against [IS leader Abu Bakr] al-Baghdadi's gangs, IS and whoever supports it," the commander said in the statement.

He went on to ask fighters who had joined IS to desert the group, called for assistance from the international community and finished by saying that the coalition wished to free all areas of Syria under IS control, including Qaraqozaq, Jarabulus, Sirin, Manbaj and Raqqa.

Syrian rebel groups have not always had a smooth relationship with Kurdish militias. The YPG, which is the main force in majority Kurdish parts of the country, has maintained an uneasy truce with President Bashar al-Assad's forces in the region, leading some rebels to accuse them of colluding with the government, a charge the Kurds deny.

However, it has been instrumental in the fight against IS and has been one of the few forces to mount an effective defense against the jihadis, inflicting heavy losses against them clashes across northern Syria and stymying their efforts to extend control into Kurdish regions. Problematically for the US, the YPG is affiliated with the Kurdistan Worker's Party (PKK), an organization which is on both America and the EU's terrorist blacklist.

The US has found itself unofficially allied with the group all the same. When IS besieged tens of thousands of members of Iraq's minority Yazidi population on Mount Sinjar, the YPG made a humanitarian corridor from Iraq into Syria allowing Yazidis to escape the mountain while American planes hit IS targets and provided humanitarian aid.

The loose coalition of moderate Syrian rebels backed by the West has not been so effective against IS, however, and lost ground to the hardline militants in recent months. The FSA and others have a complex but often cooperative relationship with more extreme groups, including al-Qaeda's official Syrian franchise Jabhat al-Nusra, which has also fought against IS.

Meanwhile, one of Syria's most powerful rebel group named new leadership on Wednesday after a massive explosion wiped out its senior figures during a meeting in Syria's Idlib province. Ultraconservative Ahrar al-Sham has effectively battled Syrian government forces and fought fiercely against IS. However the attack — which killed leader Hassan Abboud and many of his officers — raises major questions over its future. The group is a key member of IF and has close ties with al-Nusra so has been spurned by the US as a result. However it has also worked closely with more moderate revels.

A spokesperson said in a video statement that Ahrar al-Sham would now be led by Hashem al-Sheik, otherwise known as Abu Jaber and that it would continue to battle both Assad and IS. What lies next for the group is still unclear, however. IF is ideologically opposed to the FSA. It wishes to establish an Islamic state in Syria and does not recognise the exiled political opposition. However, it regularly collaborates with the FSA and others, as with the Burkan al-Forat coalition.