The US-led coalition fighting ISIL faces "tough decisions" on how much it should support Kurdish fighters in an offensive to reclaim Raqqa from the armed group, a senior US politician warned on Wednesday.

Senator John McCain said the Trump administration is weighing the thorny issue of giving the Kurds heavy arms and a major role in the upcoming battle, which would infuriate key ally Turkey.

"The conundrum is that if you don't use the Kurds, [the battle] takes a lot longer," McCain told reporters.

"But if you do, you have an enormous challenge as far as relations with Turkey are concerned, including things like the use of Incirlik," he added, referring to the Turkish airbase used heavily by the coalition to hit ISIL targets in northern Syria.

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McCain said he met recently with Turkish President Recep Tayyip Erdogan, and the two spoke about the Kurdish issue.

"There's going to be some tough decisions made here," McCain said, adding the Kurds were Erdogan's "first priority".

In August, Turkey launched Operation Euphrates Shield inside Syria targeting ISIL and the Syrian Democratic Forces (SDF), which are dominated by YPG Kurdish fighters that Ankara says are "terrorists".

The SDF is the primary ground force encircling Raqqa, which the Islamic State of Iraq and the Levant (ISIL, or ISIS) group views as its capital, and an offensive on the city is only a matter of time.

The Pentagon on Wednesday said it had provided airlift and artillery support for primarily Arab components of the SDF in a battle for Tabqa, a key town near Raqqa.

In a statement put out on social media on Wednesday, the SDF said the US-coalition had air-dropped US and SDF forces near Tabqa, expanding its campaign against ISIL in the area.

Pentagon spokesman Eric Pahon described the operation as a large, high-priority offensive to secure the area around Tabqa and the associated Tabqa Dam on the River Euphrates.

"This is a significant strategic target," he said. If successful, the operation would "basically cut ISIS off" from the western approaches to Raqqa.