Members of Roj Mine Control Organization destroy explosive devices planted by Islamic State militants near Ain Issa village north of Raqqa, Syria, June 20, 2017. [Photo/Agencies]

The incident closely followed Sunday's US downing of a piloted Syrian army jet in the southern Raqqa countryside after it dropped bombs near US-backed forces.

That was a rarity in modern warfare, representing the first time the US Air Force had downed a manned jet since 1999.

In the latest incident, the Pentagon said a US F-15 aircraft flying over Syrian territory fired on the drone after it displayed hostile intent and advanced on coalition forces.

Pentagon spokesman Navy Captain Jeff Davis said it had "dirty wings," meaning it was armed.

"I can tell you it was an Iranian-made drone," Davis said, declining to speculate on who specifically was operating it.

A Western intelligence source said it was "unquestionably Iranian".

"They are testing the limits," the source told Reuters without elaborating.

The area falls in a part of Syria that was recently identified as a military priority by Damascus, and is seen as strategically important for Iran as it seeks to secure a land corridor between forces it backs in Syria and Iraq.

The US-led coalition said the location was close to where another "pro-regime" drone, which intelligence sources had also identified as Iranian, was shot down on June 8 after dropping bombs near coalition forces.

In an indirect reference to Iranian-backed forces that have been gathering in the eastern desert region, a US-led coalition statement cited a recent escalation of tensions and said it would not "tolerate any hostile intent and action of pro-regime forces".