Hundreds of Islamic State militants have surrendered as they left the group's last enclave in eastern Syria amid more than 6,500 people, mostly civilians, who were evacuated in the last 24 hours.

Islamic State (IS) faces defeat in Baghouz on the banks of the Euphrates, but it still holds remote pockets of land further west and has launched guerrilla attacks in other areas where it has lost control.

The Syrian Democratic Forces (SDF) said on Monday they had slowed their assault on Baghouz because more civilians, previously thought to have completely evacuated, were trapped in the enclave, but they promised to capture it soon.

Mostafa Bali, head of the SDF media office, said that among some 6,500 people fleeing since Monday after the SDF opened a corridor for them to flee, hundreds of jihadists had surrendered.

"For the second consecutive day our forces had succeeded in evacuating more than 3,500 between women, children and men. There were more than 500 men mostly from foreign nationalities," Bali told Reuters.

Around 3,000 were evacuated on Monday.

The militant fighters hail from a number of countries including Uzbekistan and Turkmenistan.

The SDF, led by the leftist Kurdish-YPG militia, has also attracted fighters from radical secular Marxist volunteers from across Europe who are ideologically opposed to Islamist groups.

Baghouz, a collection of hamlets and farmland near the border with Iraq, is the last patch of populated territory that IS still holds in the area straddling the two countries where it declared a caliphate in 2014.

Meanwhile, the Syrian military has mounted air strikes against Islamic State militants and clashed with the jihadists in central Syria, the pro-Damascus al-Watan newspaper reported.

The flare-up in the area of al-Sukhna, between Palymra and Deir al-Zor, on Monday points to the foothold the ultra-hardline Islamist group still has west of the Euphrates even as US-backed fighters are poised to seize its last enclave east of the river.

The Syrian air force mounted "a number of air strikes targeting Daesh movements in the eastern Badiya, specifically on one of the dirt roads leading to the town of al-Sukhna and southeast of the town", al-Watan said, citing a military source.

Daesh is the Arabic acronym for Islamic State.