BEIRUT (REUTERS) - Small groups of Syrians are hunting down Islamic State In Iraq and Syria (ISIS) fighters in one of their main strongholds in eastern Syria in a new guerrilla campaign that has emerged as a response to the Islamists' growing brutality.

The main aim is to generate fear in the ranks of ISIS, said the head of White Shroud - a group that says it has killed more than 100 ISIS fighters in attacks in Deir al-Zor province in recent months.

The name reflects that aim: White Shroud is a reference to the death shroud it says awaits ISIS fighters responsible for crimes against the Syrian people, said the group's leader, Abu Aboud, in an interview via Skype.

As the United States advances plans to train and equip the moderate opposition to President Bashar al-Assad as part of its strategy to tackle ISIS, the appearance of such groups shows how it has generated new enemies on the ground.

Abu Aboud, who declined to give his real name for security reasons, was a commander in an anti-Assad insurgent group crushed by the better armed and financed Islamic State as it seized almost full control of Deir al-Zor earlier this year.

The small band he now leads is in no position to deal a major blow to Islamic State. But it does pose an extra challenge as the United States and its allies target the group in air strikes in both Syria and Iraq.

The Syrian Observatory for Human Rights, which tracks the war, has recorded a rising number of attacks by gunmen on ISIS targets in Deir al-Zor province. Together with Raqqa province further north, Deir al-Zor forms the bedrock of Islamic State's influence in Syria.

White Shroud shows no mercy to Islamic State: when it manages to abduct one of its members, it is only to "liquidate"him later on, said Abu Aboud.

It operates in and around the town of Al Bukamal at the Iraqi border - an area of crucial importance to ISIS as the link between the territory it controls in Syria and Iraq. The group currently numbers 300 members, said Abu Aboud.

"Eighty per cent of the members of White Shroud did not take part in combat before (ISIS) came," he said. "We trained them and they joined White Shroud because of the great oppression they felt after Islamic State took control."

The Syrian Observatory for Human Rights says White Shroud is one of several small groups that have taken up arms against Islamic State in Deir al-Zor province in recent months and are picking off ISIS fighters whenever they get the chance.

They have all taken similarly menacing names. These include the Phantom Brigade and The Brigade of the Angel of Death, said Rami Abdulrahman, who runs the Observatory, which says it gathers information from sources on all sides of the conflict.

One such group killed no fewer than 10 Islamic State fighters in a nighttime gun attack on a checkpoint in the town of Al Mayadin in Deir al-Zor province last Thursday, the Observatory reported. In a separate attack, a gunman on a motor bike opened fire on another Islamic State checkpoint.

"There is an increase in their operations against Islamic State," Abdulrahman said.

ISIS has made plenty of enemies during its conquest of Deir al-Zor, an oil-producing region.

It expelled most other insurgent groups from Deir al-Zor in July, emboldened by rapid gains in Iraq where it seized the city of Mosul in June, capturing with it Iraqi army equipment that has been deployed in Syria.

Mirroring its approach elsewhere, ISIS has used crucifixions and decapitations to suppress all opposition in Deir al-Zor. It executed 700 members of one rebellious tribe, the Sheitaat, in August, the Observatory reported.

Members of anti-Assad armed groups loosely referred to as the Free Syrian Army had the choice of fleeing, submitting to ISIS rule, or death. The Nusra Front, al Qaeda's official affiliate in Deir al-Zor, withdrew from the province.

The Syrian government still controls a portion of Deir al-Zor city and its airport.

"Secrecy is the most important element of White Shroud's work," said Abu Aboud. The group comprises four-man "cells" that work independently of each other, Abu Aboud said.

One of White Shroud's biggest operations was an attack on an ISIS position in Al Bukamal in which around 11 ISIS fighters were killed, according to the Observatory and Abu Aboud.

The U.S.-led air strikes are not making White Shroud's job easier, said Abu Aboud. Where ISIS fighters once used to gather in large numbers, they now move in small groups, often at night, using motor bikes.

White Shroud's spokesman said the group is using weapons that formerly belonged to anti-Assad rebel groups.

The spokesman, who gave his name as Abu Ali Albukamali, said that despite its modest resources, White Shroud had achieved its goal: "The aim of this group - spreading fear among Islamic State members - has been realised. Today, you never meet them walking alone. They mostly move in groups, afraid of abduction."