Islamic State (IS) militants were putting up a last bid fight in their remaining Syrian enclave of Raqqa as the US-backed coalition drives them out of the city which they have controlled since 2014.

Colonel Ryan Dillon, spokesman of the US Combined Joint Task Force-Operation Inherent Resolve, said that ISIS resistance was increasing, while the Syrian Democratic Forces (SDF) continued the fight in Raqqa.

"In the first two weeks, there was significant progress — very quick progress that was made," he said.

However, he said that now there was more significant resistance by ISIS militants in Raqqa, the Kurdish news agency ARA reported on Monday.

"The SDF in the advance have since hit some significant resistance from ISIS. And now we are starting to see some of these, you know, better-emplaced defences as we have gotten to the places where we have advanced to in the campaign in and around Raqqa," said Dillon.

"Our partnered force in Raqqa, the SDF are in their third week of offensive operations to unseat terrorists from ISIS' de facto capital. The SDF have taken 45 square kilometres of ground from ISIS, in and around Raqqa, this past week.

"The SDF continues to fight, along with three axes, towards the centre of Raqqa against substantial ISIS resistance," he stated.

Moreover, the coalition spokesperson said that it was still unclear where the US-led coalition would fight next after Raqqa is liberated.

"Once we call the liberation, or once Raqqa has been liberated, then we have to see, then, where else is there to go, where else is there ISIS-held territory," he said.

"Right now, we know that ISIS-held territory is along the middle Euphrates river valley. But is that going to still be true at the end of the fight in Raqqa? We're going to have to see about that," added Dillon.

The official added that it was possible they would continue to combat ISIS along the Euphrates river valley.

"So we will have to evaluate and see where ISIS still holds territory after the fight of Raqqa. Clearly, right now, that's along the middle Euphrates river valley.

"We will continue to strike resources and leaders throughout that middle Euphrates river valley — in Mayadin, Abu Kamal, Al-Qaim," Dillon added.

In the wake of a military victory, the US wants local governance in northern Syria, despite being uncertain as to what Syrian President Bashar Al Asaad, or the SDF would do after Raqqa was liberated.

"I know that we are consistently pushing towards local governance, which is the councils and the security forces that liberate areas from ISIS, who are representative and responsive to the people in those areas," said Dillon.

However, a US state department spokesperson said it was unclear yet what the post-ISIS plan was.

"So we aren't completely there yet. The fight for Raqqa is still underway and still taking place. The United States will never determine who will take control or take charge in terms of its government. That will be up for the Syrian people to decide," US state department spokesperson Heather Nauert stated.

Moreover, she refused to answer if the US would continue to support the SDF after ISIS is defeated in Raqqa.

"The SDF has been an effective fighting force, and we see the SDF as the best force to take back control over Raqqa, for example. And we've worked with them closely, certainly, in that arena.

"And they've done a very good job. But beyond that, I'm not going to characterise or get into hypotheticals about the future," added Nauert.