KABUL: Thousands of protesters marched coffins containing the decapitated bodies of seven Shia Hazaras through the Afghan capital Kabul Wednesday to demand justice for the gruesome beheading's, which the United Nations (UN) say may be considered a war crime.

Demonstrators gathered in the rain in west Kabul and marched towards the city centre, chanting death slogans to the Taliban and the self-styled Islamic State (IS) group while demanding justice and protection from the government.

Though it is unclear who is responsible, both the Taliban and IS affiliates have been blamed for the beheading's, which have prompted fears of sectarian bloodshed in the war-torn country.

The marchers carried pictures of the victims, including two women and one child — a girl, whose coffin was carried by grieving women.

"This is a protest to demand justice for the victims who were so mercilessly murdered, we demand justice for people who are brutally killed by terrorists everyday," protester Mohammad Hadi told AFP.

"We want revenge, today they kill us, tomorrow they kill you," the protesters chanted.

The protesters plan to take the bodies, which were brought to Kabul Tuesday night after being found in restive Zabul province, to the presidential palace, organisers said.

The protest came as the UN followed the Afghan government and the United States (US) in condemning the killings, suggesting they may have been a war crime.

"These senseless murders may amount to war crimes and the perpetrators must be held accountable," Nicholas Haysom, the UN's Special Representative for Afghanistan, said in a statement Wednesday.

The circumstances surrounding the beheadings remain unclear.

The bodies of the seven victims, who are believed to have been held hostage by unknown gunmen for months, were found on Saturday in Zabul province, where fighting between rival Taliban groups has escalated over recent days.

Some local officials have attributed the macabre killings to IS sympathisers, however the government does not have control of the area and the claim could not be verified.

Afghanistan's intelligence agency on Tuesday rejected the suggestion that IS affiliates were responsible, saying that the southern province has been the scene of deadly clashes between the rival Taliban factions for days.

The three million-strong Afghan Hazara community has been persecuted for decades, with thousands killed in the late 1990s by the mainly Pashtun Sunni Taliban.

There has been an upsurge in violence against the mostly-Shia Hazara this year, with a series of kidnappings and killings that have triggered a wave of fury on social media and condemnation by the Afghan president and the US.