Trump asks Europe take back jihadists, as IS in last stand in Syria

US President Donald Trump Sunday called on his European allies to take back alleged jihadists captured in Syria, as Islamic State group fighters defended the last scrap of their "caliphate".

IS declared a proto-state across parts of Syria and neighbouring Iraq in 2014, but has since lost all of it except a tiny patch of less than half a kilometre square near the Iraqi border.

After years of fighting IS, the Kurdish-led Syrian Democratic Forces (SDF) hold hundreds of foreigners accused of fighting for the group, and well as related women and children.

Syria's Kurds have repeatedly called for their countries of origin to take them back, but these nations have been reluctant.

The issue has taken on greater urgency, however, amid fears of a security vacuum since Trump's shock announcement in December that US troops would withdraw.

"The United States is asking Britain, France, Germany and other European allies to take back over 800 ISIS fighters that we captured in Syria and put them on trial," Trump said in a tweet, using another acronym for IS.

"The Caliphate is ready to fall. The alternative is not a good one in that we will be forced to release them. The US does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go."