The US mission in Syria is not over with the dramatic killing of the Islamic State leader, Abu Bakr al-Baghdadi, the Pentagon says, Eurasia Diary reports citing Associated Press.

The Pentagon is increasing US efforts to protect Syria's oil fields, even as American troops are withdrawn from other parts of the country.

Defence Secretary Mark Esper and Gen. Mark Milley, chairman of the Joint Chiefs of Staff, spoke at a Pentagon news conference to cheer the successful mission by US special operations forces that ended with al-Baghdadi blowing himself up.

Esper called al-Baghdadi's death a "devastating blow" to an organisation that already had lost its hold on a wide swath of territory in Syria and Iraq.

Defence Secretary Mark Esper says the military's ongoing mission to protect oil fields will ensure income for Syrian Kurds who are counted on by Washington to continue guarding Islamic State prisoners and helping American forces combat remnants of the group .

Milley said the US had disposed of al-Baghdadi's remains "appropriately" and in line with the laws of armed conflict.

Esper hinted at uncertainty ahead in Syria , even though the Islamic State has lost its inspirational leader, with the Syrian government exploiting support from Russia and Iran.

"The security situation in Syria remains complex," Esper said.

A big part of that complexity is the rejigging of the battlefield since Trump earlier this month ordered a full US troop withdrawal from positions along the Turkish border in northeastern Syria.

Even as those troops leave, other US forces are heading to the oil-producing region of eastern Syria, east of the Euphrates River.

Trump recently has proposed hiring an American oil company to begin repairing Syria's oil infrastructure, which has been devastated by years of war. Repeated US airstrikes against facilities for oil storage, transport, processing and refining starting in 2015 inflicted heavy damage.

Esper emphasised that the purpose of securing Syria's oil region is to deny income to the Islamic State.

In recent days, , US officials detected what they considered to be a significant massing of Syrian and Russian forces on the western side of the Euphrates River near Deir el-Zour, a US official said Monday. Russian officials were contacted by phone, and the US was given assurances that the staged forces would not move east, the official said.