Islamic State members now have more to worry about than U.S.-led airstrikes and the Kurdish peshmerga fighters, with the recent increase in guerrilla attacks against them conducted by several Syrian rebel combat groups in eastern Syria.
The Syrian Observatory for Human Rights, a monitoring group tracking the conflict, reports that they have seen an increase in the amount of guerrilla attacks on ISIS militants by gunman in the Deir al-Zor province, which alongside the Raqq province to the north, constitutes the biggest cog in the Islamic State threshold in Syria.
The Observatory said there are a number of groups that are conducting attacks on ISIS within the province, which is a major oil region in Syria and is still partly controlled by the government.
Many of the groups have creative and threatening names. One group, which calls itself "White Shroud", claims to have already killed over 100 ISIS fighters in attacks in the Deir al-Zor in recent months, The Daily Mail reports.
The White Shroud, which references the shroud that ISIS fighters will wear upon their death, is made up of members from a defunct anti-Assad rebel faction that was defeated earlier this year when ISIS seized a majority of the province. The group's leader, who uses the pseudonym Abu Aboud, says that there are currently 300 members in the group and the group is growing.
The group's spokesman, Abu Ali Albukamali, said White Shroud's goal is to spread fear amongst Islamic State militants.
"The aim of this group - spreading fear among Islamic State members - has been realized," Albukamali said. "Today, you never meet them walking alone. They mostly move in groups, afraid of abduction."
The group operates near the Iraqi-Syria border in the town of Al Bukamal and uses weapons they took from Assad's forces. Aboud states that one of the group's most successful attacks occurred at an ISIS-held position in Al Bukamal in which they killed 11 militants.
Although the U.S.-led coalition is trying to help defeat ISIS by conducting airstrikes, Aboud said the airstrikes have made White Shroud's operations tougher to accomplish as they can no longer move in large groups and have to move in small groups during the night. Aboud said that White Shroud operates in squads consisting of four men. Each squad operates separately from the others.
"Secrecy is the most important element of White Shroud's work," Aboud said.
Although the group is founded by fighters who previously fought against Assad forces, Aboud said that many of the newer members of the group never took part in combat before ISIS rose to prominence.
"Eighty percent of the members of White Shroud did not take part in combat before ISIS came," Aboud said. "We trained them and they joined White Shroud because of the great oppression they felt after Islamic State took control."
The head of the Observatory, Rami Abdulrahman, said that several other guerrilla groups in the area have equally creative and intimidating names such as "Phantom Brigade" and "The Brigade of the Angel of Death."
"There is an increase in their operations against Islamic State," Abdulrahman said.
The Observatory also reported that last Thursday guerrilla fighters from an unknown group killed at least 10 ISIS fighters manning a nighttime checkpoint in the town of Al Mayadin. The group also reported that one guerrilla fighter riding a motorbike shot and killed another ISIS militant.