By Mariam Karouny

BEIRUT (Reuters) - A U.S. special forces raid against an Islamic State leader in Syria caught the jihadist group off guard, killing not only the declared target, but also two other important figures, jihadist sources in Syria said.

The sources said a spy must have infiltrated the movement and passed on vital information that helped the U.S. commandos zero in on the home of their victim early Saturday when most of the guards had left to join a battle elsewhere.

They said the ultra-harldine group had absorbed the shock, but promised that any culprits would be discovered. The Islamic State was also considering tightening its recruitment procedures to try to root out moles and was considering forming a specialist unit to counter such attacks in future.

"This is a lesson for us. We consider what happened as a lesson not to underestimate our enemy regardless who he is," said one of the group's fighters inside Syria reached by Reuters via the Internet, who declined to be named.

The fighters are not allowed to speak to the media and face severe punishment if they flout the rule.

U.S. Delta Force reached deep into eastern Syria in the early hours of Saturday for their ground assault, departing from their usual reliance on air strikes alone to hit the Islamic State, which holds swathes of both Iraq and Syria.

During the raid, the U.S. troops killed Abu Sayyaf -- a Tunisian citizen whom Washington believes was responsible for overseeing Islamic State's financial operations and was involved in the handling of foreign hostages.

Islamic State has yet to make any formal statement about the attack in Deir al-Zor province, and it appears to be business as usual in the territory it holds. A resident in the northeastern Syrian city of Raqqa -- the group's de facto capital -- said life continued as before.

Sources told Reuters that two other leaders died in Saturday's incursion -- Abu Taym, a Saudi believed to oversee oil operations in the area, and Abu Mariam, who worked on group communications. His nationality was not immediately known.

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Abu Sayyaf's two brothers were wounded and his wife, who is believed to have overseen a slave market for abducted Yazidi women, was captured and flown back to Iraq.

"The reason this has happened is because of the spies. Someone from inside has helped them," said a fighter within Syria, who asked not to be named for security reasons.

"They knew exactly where to go and when. They went to the building where he was staying with his family. They did it at a time when we have minimized the guards around the compound because they were sent to a battle," he said.

RESTRICTIONS ON RECRUITS

Abu Sayyaf and his family were staying in a compound that contained at least 50 buildings, each four storeys high, where 1,000 people including civilians, lived.

The compound was built by the Syrian government to accommodate families of employees and engineers who run the nearby al-Omar gas and oil plant.

When Islamic State seized the area last year, it kept only a few dozen government employees, enough to operate the plant. The rest were killed or expelled and their houses handed over to Islamic State fighters and their families.

"The (Islamic) State is now taking new measures. One of those measures is to increase restrictions on joining. Members will be reviewed and new ones will have to be recommended. Whoever they are," said a Syrian Islamic State fighter from inside Syria.

Earlier this month, Islamic State issued an audio recording that it said was by its leader Abu Bakr al-Baghdadi, calling on supporters around the world to join the fight in Syria and Iraq. Many hundreds of foreign fighters have swelled the group's ranks and it was not clear if these new measures would slow the flow.

Abu Sayyaf has been quietly replaced in the group hierarchy and there were no signs that his death had had a direct impact on its current battles or the movement's structure.

Just hours after the U.S. sortie, Islamic State fighters overran the Iraqi provincial capital of Ramadi dealing a major blow to Iraq's government and its Western backers. In Syria, it pressed on with its assault on the ancient city of Palmyra.

Fighters and jihadi sources say the group is built in such a way that it can easily absorb the deaths of leading figures.

"We are here to die, we are here to become martyrs. Even our Caliph could be a lucky martyr one day so even if this happens, the State will not collapse. It has become bigger than one person," said another fighter from a Middle East country.

STRIKING THE EGO

Fighters contacted by Reuters inside Syria were initially stunned that such a raid could have happened and its loyalists on social media have made little or no mention of the incident.

The group takes pride in being impenetrable to foreign intelligence services, particularly in Syria, believing it can root out infiltrators before they can cause any damage.

Once caught, suspected spies are often executed in public, with videos of the beheadings or shootings regularly posted on the Internet to deter would-be agents. Their bodies are sometimes left out for days as an example for others.

Communication with media is also rare and controlled.

Fighters believe that such restrictions have allowed the organisation to operate quietly and effectively, regularly catching its enemies unawares with surprise offensives.

This also helps explain, they said, the failure of a similar U.S. raid to rescue American hostages last summer.

"We knew it was going to happen then. We quietly evacuated the place. They came, there was no one," a Syrian fighter, who said he had been in Raqqa then, told Reuters.

"But this time they were successful. It is spies, but they will be found and punished in no time. As for us, we will continue our path, the path of jihad."

(This story has been refiled to correct spelling of flout in paragraph five)

(Editing by Crispian Balmer)