Baghouz (Syria) (AFP) - Dozens of Islamic State group jihadists emerged from tunnels to surrender to US-backed forces in eastern Syria on Sunday, a day after their "caliphate" was declared defeated.

Syria's Kurds warned that despite the demise of the proto-state, the thousands of foreign jihadists they have detained are a time-bomb the world urgently needs to defuse.

An AFP reporter saw dozens of people -- mostly men -- file out of the battered jihadist encampment in the remote village of Baghouz near the Iraqi border to board pickup trucks.

"They are IS fighters who came out of tunnels and surrendered today," Kurdish spokesman Jiaker Amed said.

Some sported thick beards and wore long woollen kaftans over their dark-coloured robes, or a chequered scarf around their faces, as they trudged out of their final hideout under the drizzle.

"Some others could still be hiding inside," said Amed.

World leaders were quick to hail Saturday's announcement by the Kurdish-led Syrian Democratic Forces that the last shred of land controlled by IS in Syria had been conquered.

But the top foreign affairs official for the country's semi-autonomous Kurdish region warned IS members captured during the assault still posed a threat.

"There are thousands of fighters, children and women and from 54 countries, not including Iraqis and Syrians, who are a serious burden and danger for us and for the international community," Abdel Karim Omar told AFP.

"Numbers increased massively during the last 20 days of the Baghouz operation," he said.

He also warned of the continuing danger posed by IS sleeper cells.

The SDF is continuing to carry out operations to rout out any remaining jihadists in the area and uncover possible weapons caches.

"This back-clearance operation will be deliberate and thorough and help ensure the long-term security for the area," the US-led coalition backing the SDF wrote on Twitter.

- 'Future terrorists' -

As the SDF's months-long assault closed in against the last IS strongholds in the Euphrates Valley, jihadists and their families gradually gathered in Baghouz.

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While some managed to escape, many foreigners stayed behind, either surrendering or fighting to the death.

According to the SDF, 66,000 people left the last IS pocket since January, including 5,000 jihadists and 24,000 of their relatives.

The assault was paused multiple times as the force allowed people to evacuate from the enclave on the banks of the Euphrates.

The SDF have screened droves of people scrambling out of Baghouz in recent weeks, detaining suspected jihadists and trucking civilians and IS relatives to camps further north.

Most relatives have been crammed into the Al-Hol camp, a facility built for 20,000 people but which now shelters 72,000.

The Kurdish administration in northeastern Syria has warned it does not have capacity to detain so many people, let alone put them on trial.

But the home countries of suspected IS members are reluctant to take them back, due to potential security risks and the likely public backlash.

Several held in Syria have been stripped of their citizenship.

"There has to be coordination between us and the international community to address this danger," Abdel Karim Omar said.

"There are thousands of children who have been raised according to IS ideology," he added.

"If these children are not re-educated and re-integrated in their societies of origin, they are potential future terrorists."

- 'New phase' -

The SDF's main support has been the international military coalition launched by the United States in mid-2014 to counter the expansion of IS.

Its aerial campaigns against IS hubs across a "caliphate" which once spanned territory the size of the United Kingdom have levelled major cities and contributed to the biggest wave of displacement since World War II.

According to the Airwars monitoring NGO, at least 7,500 civilians have died as a result of coalition actions in four and half years.

The conflict in Syria has killed more than 370,000 people since it erupted eight years ago, the Syrian Observatory for Human Rights war monitor says.

US President Donald Trump has vowed to drastically scale down US military presence in Syria once IS is defeated, leaving the Kurds exposed to threats by Damascus and Turkey.

Ankara sees the SDF as a terrorist organisation and Abdel Karim Omar warned that any cross-border offensive risked leading to mass breakouts from the jails where jihadists are currently held.

"Any new threat or new war would give an opportunity to these criminals to slip out," he said.

IS, faced with multiple offensives in Syria and Iraq since 2014, has morphed from a territorial force back into a clandestine insurgency group carrying out hit-and-run attacks in both countries.

The SDF's top commander said Saturday that anti-IS operations were entering a new phase.

Mazloum Kobane said the new focus would be IS sleeper cells that "are a great threat to our region and the whole world."