BAGHDAD - The Islamic State group strung up four Iraqi fighters with chains and burned them alive, according to footage posted online, the latest gruesome execution video from the militants.

The victims - identified as fighters in the pro-government Popular Mobilisation forces from southern Iraq - were suspended from a swingset-like metal structure by chains attached to their hands and feet, then set on fire.

IS, which overran large parts of Iraq last year and still controls much of the country’s west, said the murders were in revenge for the alleged burning of four men by pro-government forces.

“Now retribution has come, for today, we will attack them as they attacked us, and punish them as they punished us,” a masked militant says in the video.

The video was not dated and did not give a specific location for where the killings took place, but it did carry a tag indicating that it was produced by the IS media unit responsible for Iraq’s Anbar province.

The video included a clip said to show a man suspended over a fire while still alive as pro-government forces look on, and another of famous fighter Abu Azrael (“Father of the Angel of Death”) slicing a piece of flesh off a burned corpse with a sword. IS has carried out a slew of atrocities in territory it controls in Iraq and Syria, such as mass executions and a campaign of killings, kidnapping and rape targeting minorities. It has recorded many killings - including beheadings, shootings, drownings and burnings - in videos posted online.

Baghdad’s forces regained significant ground from the militants in two provinces north of the capital with support from a US-led coalition and Iran, but much of western Iraq remains outside government control.

Meanwhile, the Islamic State group battled Syrian rebel forces in a Damascus neighbourhood on Monday, bringing the militants closer than ever to the centre of the capital, a monitoring group said.

IS militants fought street battles against Islamist rebels in Asali, part of the capital’s southern Qadam district, after seizing two streets there over the weekend, the Syrian Observatory for Human Rights said.

“This is the closest IS has ever been to the heart of Damascus,” Observatory head Rami Abdel Rahman said. He said the militants had advanced from the adjacent Al-Hajar Al-Aswad neighbourhood, where they have been based since July 2014. A Syrian military official confirmed the clashes and said he was “very happy that they are fighting.” “But we are ready to react if they try to advance into government-held territory,” the official told AFP. According to the Observatory, opposition-held Qadam has been relatively quiet since a truce between rebel groups and regime forces there a year ago.

It said fighting in the district on Sunday left 15 fighters dead, but it could not specify how many were from IS and how many were Islamist rebels.

Abdel Rahman said the “fierce street battles” had forced civilians to flee the area.

Since its expulsion from the Eastern Ghouta suburb of Damascus last year, IS has used Al-Hajar Al-Aswad as a base for attacks on the capital.

From there, it tried to seize the Yarmuk Palestinian refugee camp in April, but was pushed back.

That same month, IS kidnapped two opposition fighters from Qadam and beheaded them in Al-Hajar Al-Aswad.

More than 240,000 people have been killed in Syria’s conflict, which began with popular anti-government protests in March 2011 but has evolved into a complex civil war.

The conflict has seen the embattled regime of President Bashar al-Assad lose swathes of territory across the country.

In the northwest province of Idlib, the powerful Army of Conquest alliance edged closer to Fuaa, one of two remaining regime-held villages in the province.

The Observatory said the alliance, a collection of Islamist and militant groups including Al-Qaeda’s Syria affiliate, seized the village of Sawaghiya on the southeast edge of Fuaa early Monday after overnight clashes.

The fighting left nine fighters from both sides and two civilians dead.

After capturing the majority of Idlib province, the Army of Conquest surrounded and began heavily shelling the villages of Fuaa and Kafraya.

This month saw two failed attempts at reaching broad ceasefire deals including Fuaa, Kafraya, and the rebel stronghold of Zabadani in Damascus province.