By John Davison and Ellen Francis
AIN ISSA, Syria/BEIRUT (Reuters) - U.S.-backed militias launched their "final" assault on Syria's Raqqa on Sunday after letting a convoy of Islamic State fighters and their families quit the city, leaving only a hardcore of jihadists to mount a last stand.
"The battle will continue until the whole city is clean," said a statement by the Syrian Democratic Forces (SDF), a U.S.-backed alliance of Kurdish and Arab militias.
SDF spokesman Talal Selo told Reuters that "no more than 200-300" foreign militants remained to fight on in the city after the convoy left. "This is the final battle," he said.
Under the withdrawal deal between Islamic State and tribal elders, the jihadists would let all other civilians trapped in Raqqa have safe passage out of the city, he said. Selo added that he believed only a few may have remained.
Raqqa's fall to the SDF now looks imminent after four months of battle hemmed the Islamic State jihadists into a small, bomb-cratered patch of the city.
"We still expect there to be difficult fighting," said Colonel Ryan Dillon, spokesman for the U.S.-led international coalition backing the SDF in the war against Islamic State with air strikes and special forces. The coalition will continue to operate on the basis that civilians remain in Raqqa, he said.
Raqqa was the first big Syrian city that Islamic State seized as it declared a "caliphate" and rampaged through Syria and Iraq in 2014, becoming an operations center for attacks abroad and the stage for some of its darkest atrocities.
But Islamic State has been in retreat for two years, losing swathes of territory in both countries and forced back into an ever-diminishing foothold along the Euphrates river valley.
"Last night, the final batch of fighters (who had agreed to leave) left the city," said Mostafa Bali, another SDF spokesman.
There were conflicting accounts as to how many people left in the convoy.
Selo said 275 Syrian militants left along with their family members. Laila Mostafa, head of the Raqqa Civil Council formed under SDF auspices to oversee the city, said that figure included both the fighters and their family members. In a statement, she denied an earlier comment by another council member that some foreign fighters had left in the convoy.
Before the evacuation, the coalition estimated that about 300-400 fighters were still holed up in the Islamic State enclave.
Fighters who left in the convoy, which the coalition is tracking, had given biometric data including fingerprints, Dillon said.
The convoy was still in territory held by the SDF on Sunday morning, Selo said.
Bali described the civilians who left with Islamic State fighters in the convoy as human shields. The jihadists had refused to release them once they left the city as agreed, wanting to take them as far as their destination to guarantee their own safety, he said.
Such withdrawals of fighters along with groups of civilians have grown commonplace in Syria's six-year war, as a way for besieging forces to accelerate the fall of populated areas.
The convoy would head to the remaining Islamic State territory in eastern Syria, Omar Alloush of the Raqqa Civil Council had said on Saturday.
The agreement was brokered by the council and tribal elders to "minimize civilian casualties", the coalition has said. Tribal leaders from Raqqa said they sought to prevent bloodshed among civilians still trapped in the city.
"If there are any civilians remaining (in the enclave) they would be the families of those foreigners. The civilians exited completely," Selo said on Sunday.
The SDF's decision to hasten the battle's end by allowing Islamic State fighters to leave Raqqa was at odds with the stated wishes of the U.S.-led coalition that backs the militias.
Dillon said it was not involved in the evacuation but added: "We may not always fully agree with our partners at times. But we have to respect their solutions."
In August, the coalition spent weeks preventing a convoy of Islamic State evacuees from an enclave on the Syrian-Lebanon border from reaching jihadist territory in eastern Syria.
The SDF launched the battle for Raqqa on June 6 after a months-long campaign to isolate the city against the north bank of the Euphrates.
Islamic State, then known as Islamic State in Iraq and the Levant, had captured the city in January 2014, seizing it from rebel factions which had ousted the Syrian army a few months earlier.
As the group became more entrenched in Syria and Iraq leading up to its capture of Mosul in June that year, Raqqa became its most important center, and it celebrated its series of victories with a massive parade through the city.
Many of its top leaders were at times based there, and former hostages said Mohammed Emwazi, better known as Jihadi John, imprisoned them along with those he later executed, in a building near an oil installation near the city.
The group killed dozens of captured Syrian soldiers there in July 2014 and it was also the site of a slave market for Yazidi women captured in Iraq and given to fighters.
The coalition has said Raqqa was a hub for attacks abroad, and in November 2015, after militants killed more than 130 people in Paris, France launched air strikes on Islamic State targets inside Raqqa.
But the group is now in disarray. In Syria it does not only face the U.S.-backed SDF offensive but a rival one by the Syrian army supported by Russia, Iran and allied Shi'ite militias.
A Syrian military source said on Saturday the army had captured the city of al-Mayadin in the Euphrates valley, leaving Islamic State only a few more towns and villages, and surrounding desert territory, in Syria.
But the battle for Raqqa has come at great cost to its people. Intense coalition air strikes and the months of street-to-street fighting have pulverized much of the city. Thousands of people have fled as refugees and hundreds of civilians have died.
(Reporting by John Davison in Syria and Ellen Francis in Beirut; Writing by Angus McDowall; Editing by Janet Lawrence and Mark Potter)