British anti-IS fighters vow to avenge Manchester bombing victims

British volunteer fighters combating the Islamic State group in Syria have vowed to avenge the victims of Monday's deadly suicide attack at a pop concert in Manchester.

The British combatants, who have been fighting among the ranks of the Kurdish People's Protection Units [YPG] in Syria, promised on Tuesday to soundly defeat the extremists in their de-facto capital of Raqqa.

IS has claimed responsibility for the the massacre that killed 22 people including one girl aged just eight at a concert by US pop star Ariana Grande.

"We're taking it to them. When I go to Raqqa I'm giving them no quarter. I will expect no mercy from them and I will not give them no mercy," said Michael Enright, a YPG fighter from Manchester.

"I will remember Manchester," he swore in a video posted on Twitter.

He added that he had become accustomed to death while fighting in Syria but the attack targeting children and teenagers in his hometown was "heartbreaking".

Macer Gifford, another YGP volunteer, said in an online statement that IS would soon be crushed in Raqqa, adding that the British government must back Kurdish forces their fight.

"We will destroy their military capital which will effectively cut off the snake's head," Gifford said.

"We must urge the UK government to do more to defeat [IS] in Syria. Terror has been brought to London and Manchester. Our children have been targeted!"

He added that British authorities should follow the lead of the US and send military aid to the YPG's ally the Syrian Democratic Forces [SDF].

In November last year, the SDF - a Kurdish-Arab alliance - began an offensive aimed at taking the city of Raqqa.

The fighters are still 40 kilometres from Raqqa to the west, and have not controlled any territory directly to the city's south, which is bordered by the Euphrates river.

The SDF has said the long-awaited attack on Raqqa would start at the beginning of the summer, probably in June.

The Kurdish forces have received a steady stream of recruits from the West to help in the fight against IS extremists.

In December last year, a 20-year-old British YPG fighter was killed during the ongoing Raqqa offensive, making him the third Briton to die fighting IS in Syria.

Critics have however cautioned that the YPG and its allies are committing human rights abuses, and pursuing an agenda for Kurdish independence including by collaborating with the Syrian regime, despite its atrocities that eclipse those of IS.