While some are motivated by a genuine desire to help, others have come seeking personal glory, fighters say.
Attacks on energy facilities in northern Iraq killed at least five, including a top police commander, and wounded others.
In the first attack on Sunday on the AB2 gas compressor station, northwest of the oil-rich Kirkuk province, sources said gunmen used hand grenades to storm through the external door of the facility before shooting dead four employees in a control room.
They then planted explosives, around five of which went off, the sources said.
Counterterrorism forces regained control of the facility and freed 15 other employees who had hidden in a separate room.
The fighters could not be found and may have escaped to launch a second attack 25 kilometres away, at the Bai Hassan oilfield.
In that second attack, gunmen used the same approach to enter the facility before blowing up an oil storage tank inside.
The head of the oil police force in Kirkuk province was killed and three police officers were wounded when they responded to the Bai Hassan attack.
Security officials said a suicide bomber there blew himself up, setting off fires in two oil tanks.
ISIL, also known as ISIS, claimed responsibility for the second attack via its Amaq website without mentioning the first strike.
The oil-rich Kirkuk province is divided between areas mainly controlled by the Kurdish autonomous region and areas held by ISIL. It is at the centre of a territorial dispute between the Kurds and the central government in Baghdad.