BEIRUT -- The United States on Monday called on other nations to repatriate and prosecute their citizens who travelled to Syria to fight with the Islamic State group and who are now being held by Washington's local partners.

The Kurdish-led Syrian Democratic Forces say they have detained more than 900 foreign fighters during their U.S.-backed campaign against ISIS in northeastern Syria, where they are currently battling to drive the extremists from their last tiny pocket of territory.

The question of what to do with the detained foreigners has grown increasingly thorny since U.S. President Donald Trump's surprise announcement in December that he intends to withdraw all American forces from the country.

"The United States calls upon other nations to repatriate and prosecute their citizens detained by the SDF and commends the continued efforts of the SDF to return these foreign terrorist fighters to their countries of origin," U.S. State Department deputy spokesman Robert Palladino said in a statement.

The statement came as the SDF announced the capture earlier this month of three alleged IS fighters, from Germany, Saudi Arabia and Egypt. In addition to the hundreds of militants, the SDF are also holding more than 4,000 family members of IS fighters.

Very few countries have expressed readiness to repatriate their citizens, posing a dilemma for the Kurdish-led forces, particularly after the US said it plans to withdraw.

Last week, France's Interior Minister Christophe Castaner told French media that a handful of French jihadis had already returned home and more would follow soon after the departure of American troops. Britain refuses to take back citizens who joined IS and has reportedly stripped them of their citizenship. Other European countries have remained largely silent about the fate of men and women whom many see as a security threat.

Palladino commended the SDF's efforts and said the force has "demonstrated a clear commitment to detain these individuals securely and humanely."

IS has lost virtually all the territory it once held in Syria and neighbouring Iraq, but Palladino said it remains "a significant terrorist threat," adding that "collective action is imperative to address this shared international security challenge."

A Defence Department Inspector General report released Monday said IS "remains a potent force of battle-hardened and well-disciplined fighters that could likely resurge in Syria absent continued counterterrorism pressure." It said the militants are still able to co-ordinate offensives "as well as operate as a decentralized insurgency."

The campaign against the extremists is currently focused on a small, remote patch of land in eastern Syria, where thousands of civilians remain holed up with the militants. The battles have slowed in recent days to allow civilians to flee from the nearly 1.5 square mile (4 square kilometre) area.

The Syrian Observatory for Human Rights, which closely tracks the conflict, said more than 36,000 people, including many foreigners and over 3,000 fighters, have trickled out of the small area in recent weeks. Most were evacuated to displaced people camps but many were also taken for interrogation and questioning.

Syrian opposition activists said Monday that the SDF killed six children and three women who were trying to flee.

An SDF spokesman did not immediately respond to request for comment on the shooting.

The Observatory said the incident occurred Saturday night, adding that the gunfire came from SDF positions. It said the civilians had paid money to smugglers to take them out of the area controlled by the extremists.

The DeirEzzor 24, an activist collective, said the incident occurred near Tanak oil field, which is close to the front line between IS and the SDF, blaming the Kurdish-led force for the shooting.

------

Lee reported from Washington. Associated Press writers Lolita Baldor in Washington and Bassem Mroue in Beirut contributed.