US-backed forces have said Islamic State group was living its "final moments" after thunderous shelling on its last scrap of land in eastern Syria prompted 3,000 jihadists to surrender.

Some die-hard IS fighters who stayed to defend the remnants of their "caliphate" struck back with a wave of suicide bombings, according to Kurdish-led Syrian Democratic Forces.

IS once ruled over millions in a swathe of Syria and Iraq, but has since lost all that territory except for a riverside slither in the village of Baghouz near the Iraqi border.

Thousands of men and women have poured out of the pocket in recent weeks, hampering an advance by the US-backed SDF, which has paused its offensive multiple times to allow evacuations.

Supported by US-led coalition air strikes against the jihadists, the SDF resumed artillery shelling on Sunday after warning IS fighters their time was up.

However, clashes have continued as the SDF worked to thwart IS counter-attacks.

According to an SDF official, jihadists were using suicide bombers but his force intercepted them before they reached their target.

The jihadists "made no progress and they were stopped," he said.

"The battle is ongoing and the final hour is now closer than ever," SDF spokesman Mustefa Bali tweeted.

Since December, about 60,000 people have left the last IS temporary fortification, according to Britain-based Syrian Observatory for Human Rights, around a tenth of them suspected jihadists.

The outpouring has sparked a humanitarian crisis in Kurdish-run camps, which are struggling to accommodate the mass influx of women and children.

The United Nations food agency appealed yesterday for urgent funding for the Al-Hol camp, which is receiving the bulk of evacuees.

At the height of its brutal rule, IS controlled a stretch of land in Syria and Iraq the size of UK.

The total capture of the Baghouz camp by the SDF would mark the end of the cross-border "caliphate" it proclaimed more than four years ago.

But beyond Baghouz, IS retains a presence in eastern Syria's vast Badia desert and sleeper cells in the northeast.

The jihadists have continued to claim deadly attacks in SDF-held territory in recent months, and the US military has warned of the need to maintain a "vigilant offensive".