Iraqi security forces defend their headquarters against attacks by Islamic State in the eastern part of Ramadi in Anbar province. Source: Reuters

The operative, known as Abu Sayyaf, was killed by a Delta Force team inserted into the Islamic State-controlled al-Omar oilfield deep in northeast Syria, marking the first such strike against the jihadis in the country by US ground forces.

Abu Sayyaf is said to have managed bootleg oil production and profits worth billions to Islamic State, giving him access to the ­organisation’s leadership and intimate knowledge of its workings.

He died in a firefight with the elite US soldiers, along with an estimated dozen Islamic State fighters who attempted to use local women and children as shields, Pentagon officials said.

His wife, identified as Umm Sayyaf, was captured and is ­believed to have been privy to his financial secrets as well as information on foreign hostages held by Islamic State. None of the 40-strong assault team was killed or injured, Washington insisted.

A Yazidi woman evidently held as a slave by the couple was freed.

The raid offset gains made by the militants as fighting steps up across north and western Iraq and Syria, and threatens to spill into Lebanon’s Bekaa Valley.

Islamic State was poised to recapture Iraq’s largest oil and gas refinery at Baiji, about 200km north of Baghdad, and was making gains in the provincial capital of Ramadi despite desperate efforts by the Iraqis to bolster defences after the main government compound was seized. Late last night, authorities said several suicide car bombs had killed 15 Iraqi security force members in attacks targeting police officers and military headquarters.

At Palmyra, Syrian troops were pushing Islamic State jihadists back, in fighting that left dozens dead and eased fears for the world heritage site. However, the 2000-year-old ruins at Palmyra remained in danger of being overrun, the Syrian Observatory for Human Rights said. Turkey said it had shot down a Syrian government helicopter that crossed into its airspace.

Although he has been described by US officials as the “oil and gas emir’’ of Islamic State, Abu Sayyaf’s ranking in the hierarchy was unclear, and his name had not previously been mentioned in ­assessments of who’s who.

If he controlled the group’s oil and gas revenues, his position would justify claims he was close to self-styled caliph Abu Bakr al-Baghdadi, who reportedly suffered incapacitating spinal injuries during a coalition air strike in March.

US congress was told in Oct­ober that Islamic State earned about $US1 million ($1.24m) a day from blackmarket oil sales, a key source of revenue after kidnapping and extortion.

The night-time raid on Abu Saffaf’s heavily protected home illustrates the effectiveness of US Special Forces used for the first time to go directly after an Islamic State figure, almost certainly on the direct order of Barack Obama.

It also shows the depth of American intelligence on the shadowy cabal that runs the organisation. This is vastly more varied than what might be supposed from the extreme version of Islamism espoused by Islamic State: the brains behind it are drawn from disaffected elements of the top-drawer bureaucracies in Iraq, Syria and the professional classes of Sunni Arab nations in the ­region, and from the senior ranks of the Iraqi military.

Abu Sayyaf and his wife were reportedly confronted in a room together, where he was killed after trying to “engage’’ the Delta strike team, according to a US account of the operation.

Umm Sayyaf is believed to have been taken to a base in Iraq for interrogation. US officials were adamant she would not be transferred to the detention centre at Guantanamo Bay in US-controlled Cuba, which Mr Obama has vowed to close.

additional reporting: AFP, AP