US Special Forces have struck inside Syria before, but only on hostage rescue missions. Last year, they tried to save James Foley, an American journalist, by raiding a location in Raqqa, the unofficial capital of Isis. But all of the hostages had been moved and Foley was murdered a month later.
The latest operation appeared to be the first in Syria where the aim was to kill or capture an Isis commander.
CBS news reported that Abu Sayyaf and his wife might have been targeted because they had information about the treatment meted out to Kayla Mueller, an American aid worker who was held captive by Isis and then killed this year. His wife, known as Umm Sayyaf, is expected to be questioned about Mueller's fate.
The New York Times reported that Abu Sayyaf was a mid-level leader and "likely is replaceable in fairly short order" but the raid illustrates "improving American intelligence" on Isis leaders. It said a Defence Department official said the operation involved close "hand-to-hand fighting". The soldiers came under fire soon after they landed in Amr, Deir Ezzor province. Officials said the forces were able to seize computers and cellphones from the site.
The US raid on Syrian soil was ordered by President Barack Obama, according to a White House statement.
Isis holds lucrative oil and gas fields in eastern Syria. Control of these assets has helped it to become one of the richest terrorist movements in history. The White House said Abu Sayyaf had a "role in overseeing [Isis'] illicit oil and gas operations - a key source of revenue that enables the terrorist organisation to carry out their brutal tactics and oppress thousands of innocent civilians". The statement said that Abu Sayyaf was "also involved with the group's military operations".
His wife was suspected of playing an "important role in [Isis'] terrorist activities". She was also being held for possible complicity in the "enslavement of the young woman" who was rescued. Thousands of women and children from the Yazidi sect have been enslaved by Isis, which openly proclaims its willingness to reimpose slavery on the areas its controls.
The US national security team - including officials from the Pentagon, the CIA and the State Department - had identified Abu Sayyaf as a key target and he was eventually tracked to the Al-Amr area of eastern Syria.
After his location was established, the officials asked Obama for authority to carry out the raid. "The president authorised this operation upon the unanimous recommendation of his national security team and as soon as we had developed sufficient intelligence," said the White House.
The Iraqi government gave its consent to the raid - but a US official said there was no prior co-ordination with the Syrian regime.
Choking off Isis' sources of revenue has been a key priority of the American-led campaign. Officials have identified the group's oil refineries as a key vulnerability and scores have been destroyed by air strikes.
But the movement's fighters are still advancing in both Iraq and Syria.
In Iraq, Isis has been able to raise the black banner of jihad over the main government building in Ramadi, the capital of Anbar province.
In Syria, Isis units have struck westwards towards the classical ruins of Palmyra, a Unesco World Heritage Site.
The terrorists regard all relics of the pre-Islamic world as inherently idolatrous. Isis fighters have looted and vandalised the ruins of Nimrud in northern Iraq, pillaging the capital of the ancient Assyrian empire.
Their latest advance raises fears that Palmyra could suffer the same fate.
The sand-coloured ruins include a colonnaded street and the temple of Baal, a deity worshipped before the birth of Islam in the 7th century.
Destroying the site would be more than a religious duty for Isis. Palmyra still serves as a rare symbol of unity in Syria, amid sectarian divisions.
As befits its position at the crossroads of several ancient civilisations, Palmyra remains of strategic importance today. It lies on the junction between highways running westwards to the cities of Homs and Damascus.
