Speculation includes the opinion this is not a significant loss for ISIS /Daesh simply because removing the head may not alter the pace of operations.
The US says its special forces have killed a senior Islamic State (IS) member and captured his wife in a rare ground raid in eastern Syria.
Abu Sayyaf helped direct oil, gas and financial operations for IS, as well as holding a military role, said a US Department of Defense statement.
It said forces tried to capture him, but he was killed after engaging them.
It is the first time the US is known to have carried out a ground operation to attack IS within Syria.
The operation was authorised by President Barack Obama and was carried out by forces based in Iraq.
US officials said Abu Sayyaf was Tunisian, with one official telling CNN he was the chief financial officer "of all of [IS]" and that the US had seized "reams of data on how ISIS operates, communicates and earns its money"
The raid came after weeks of surveillance of Abu Sayyaf, using information gleaned from a small but growing network of informants whom the C.I.A. and the Pentagon have painstakingly developed in Syria, as well as satellite imagery, drone reconnaissance and electronic eavesdropping, American officials said. The White House rejected initial reports from the region that attributed the raid to the forces of President Bashar al-Assad of Syria.
“The U.S. government did not coordinate with the Syrian regime, nor did we advise them in advance of this operation,” said Bernadette Meehan, the National Security Council spokeswoman. “We have warned the Assad regime not to interfere with our ongoing efforts against ISIL inside of Syria,” she added, using another name for the Islamic State, which is also known as ISIS.