Foreign Islamic State suspects who were captured by the UK’s allies in Syria and then exchanged in secret prisoner swap deals have been executed by the jihadists as punishment for defecting.

At least two of those involved in the deals, including a woman from Tajikistan and a French man, were killed shortly after being sent back to Islamic State of Iraq and the Levant (Isil) territory against their will, witnesses say.

The Telegraph revealed in June how hundreds of foreign men, women and children detained by the Kurdish-led Syrian Democratic Forces (SDF) were swapped in return for Kurdish fighters held by Isil.

Legal groups on Friday expressed concern that Isil suspects who were forcibly sent back to a battlefield were later executed, saying it undermined international law.

One woman named Sitora, a 24-year-old from Tajikistan whose surname is not known, was executed weeks after being returned to Isil.