Among the ruins of a street in Syria's al-Raqa, an anti-extremist fighter's call echoes from a loudspeaker: "Civilians, we are waiting for you to leave to safety."

The Syrian Democratic Forces say they are close to defeating the last "Islamic State of Iraq and Syria" (ISIS) fighters holed up in the city.

The Arab-Kurd alliance has captured most of what was once ISIS's Syrian stronghold, a city that became synonymous with the group's worst atrocities.

But thousands of civilians are feared to remain in al-Raqa, some used by ISIS as human shields according to the SDF.

In the remaining pockets held by the group, the fighters are trying to encourage civilians to flee before they move in.

Near the city's large grain silos, SDF fighters urged civilians who fear ISIS snipers to emerge from their homes.

Ahmed Abu al-Sheikh, an SDF fighter with a long beard and a black scarf wrapped around his head, monitored the situation with a radio in his hand.

He heads a special unit responsible for helping evacuate civilians.

"We came here because there are groups of civilians near this point and we got information about their exit," he said.

"There are about 70 metres separating us from the civilians. We are going to get them out of ISIS's hands," he added.

Last pockets of fighting

Tens of thousands of civilians have fled al-Raqa in recent months, some of them escaping as the SDF worked to encircle the city before finally breaking into it in June.

But the UN estimates that up to 15,000 civilians could remain in parts of al-Raqa, facing "incredibly difficult conditions", including shortages of food, water and medicine.

Fleeing is a dangerous undertaking, with ISIS snipers regularly opening fire on escaping civilians and routes strewn with mines.

Evidence of ISIS rule remains all around, including a billboard reading "jihad" that sits among the destruction.

Earth berms created by ISIS in a bid to slow the SDF's advance can be seen on many streets in the city, and on the walls the words "God is greatest" are written in black.

The SDF began its attack on the city in June on two fronts, entering from the east and then the west.

But its advances in the centre of al-Raqa now mean its fighters can cross from one side to the other directly.

"We are crossing for the first time from the western front to the eastern front because our forces now hold most of the city," said Jihan Sheikh Ahmed, spokeswoman for the "Wrath of the Euphrates" campaign to capture al-Raqa.

"We control 80% of al-Raqa and the other 20% is in the firing line of our forces," she told AFP on Thursday inside the city.

She said ISIS fighters had holed up inside the al-Raqa national hospital, the local sports stadium and several parks nearby.

"There are tunnels that connect several strategic places. We are trying to seize the national hospital but ISIS is holding civilians inside it as human shields and that is slowing our progress."

'Historic moments'

International coalition airstrikes hit near the silos and stadium on Thursday, sending clouds of grey smoke into the air above.

The sporadic sound of gun and mortar fire could be heard, but among the SDF, spirits were running high.

Some of the fighters around Sheikh Ahmed were singing and even broke into the traditional dabkeh dance to show their confidence that the battle for the city is nearing its end.

"We are in the final stage and will deliver victory to our people in al-Raqa within weeks," Sheikh Ahmed said.

"Victory is near."

SDF commander Rojda Felat said the remaining ISIS fighters in the city "no longer have the ability to launch attacks or dispatch car bombs".

"This is clear from their latest movements. We expect them all to be killed in al-Raqa."

The SDF expects to declare victory in the city within "weeks", according to Sheikh Ahmed.

"We promised our friends who were martyred this victory," said Berkhdan, a young fighter wearing a green scarf with a floral pattern wrapped around his head.

"We feel happy because we are witnessing historic moments with the fall of al-Raqa and the defeat of ISIS," he added.

"The liberation of al-Raqa will enter the history books."