One of Syria's leading rebel groups Jaish al-Fatah have demanded that members of an Islamic State group-linked militia in southern Syria surrender their arms.
Earlier in the week, the headquarters of the Yarmouk Martyrs Brigade was hit by a suicide bomber.
The attack was launched by Syria's al-Qaeda affiliate al-Nusra Front, and killed the core of its leadership.
It means that the small band of fighters, numbering no more than 2,000 men.
It has led to questions about the future of the most secretive groups linked to IS, as it comes under attack from a host of rebel groups, while its new leader - described as a veteran jihadi militant - struggles to keep the militia together.
Fighters swore an oath of allegiance to a Palestinian "emir", Abu Obeida Qahtan, who took over the reins of the group after its previous leader Abu Ali al-Baridi was killed in the bomb blast.
Although Qahtan is said to be well-respected among jihadi circles for fighting alongside al-Qaeda founders Osama bin Laden and Abdullah Azzam, the killing of its experienced founder Baridi will likely be a blow to the group.
Fighters from the group insist the militia is "bigger than one man" although its core is made up of members of the Baridi family.
"Interestingly, the group pledged allegiance to this guy as its new amir, not an allegiance pledge to the Islamic State," said Aymen al-Tamimi, a Syrian analyst who has studied the group since it emerged in 2012.
Under Baridi's leadership, the Martyrs Brigade have been accused of being an IS affiliate by Nusra, while many other rebel groups dismissed them as bandits. The truth probably lies somewhere between the two.
"Among rebel groups they had a poor reputation for corruption and not doing much on the battlefield," said Tamimi.
The analyst said the the brigade flirts with IS imagery, has adopted the group's banner in some of its propaganda, and established sharia courts and a religious police force, seemingly mimicking IS' own.
Niqab (face veils) have also been distributed in the brigade's territories - a far more conservative stance on women's dress than its Islamist rivals in Jaish al-Fatah, which includes Nusra.
Its pronouncement of other rebel groups being "apostates" and use of torture has also led Nusra to accuse the brigade of being "takfirists" linked to IS.
Yarmouk Martyrs Brigade came to the attention of the world when they kidnapped a group of UN peacekeepers in the Golan Heights in 2014, bringing accusations of extremism.
In December 2014, Nusra became suspicious that the group had sworn allegiance to IS leader Abu Bakr al-Baghdadi who became one of the al-Qaeda affiliate's sworn enemies after they were kicked out of eastern Syria by their rivals.
Al-Qaeda groups in Daraa province remained loyal to Nusra's leader Abu Mohammed Joulani, but the southern affiliates were still wary of units defecting to Baghdadi's army.
In December 2014, rumours emerged that the Yarmouk Martrys Brigade were in contact with the IS leadership.
It led Nusra to swoop in an launch an offensive against the group before they formally switched sides.
Although they were not defeated, it forced its fighters to hole up in the Yarmouk valley.
Since then Yarmouk Martyrs Brigade have been largely contained in the valley, issuing periodic threats to everyone from the Assad regime to Israel.
"The group itself will not explicitly said it has an allegiance to IS - therefore, at the present time, it is more accurate to speak of the group as pro-IS/IS-oriented," said Tamimi.
"I won't venture to say I know what's going on in their members' minds, but based on their posts and conversations with me they seem ideologically committed [to IS] now."