Sri Lankan government forces and Tamil Tiger rebels "most likely" committed war crimes including mass killings of civilians during their long conflict that should be prosecuted by a special court with international judges, the United Nations says.

Key points: Both government forces, Tamil rebels accused of killing civilians

Both government forces, Tamil rebels accused of killing civilians UN calls for "hybrid special court" with international judges

UN calls for "hybrid special court" with international judges Report urges Colombo to remove any suspects from office

Report urges Colombo to remove any suspects from office Cites heavy government shelling of "no fire zones" in last phase

Despite pledges by the new government of recently elected president Maithripala Sirisena to pursue accountability domestically, the criminal justice system is not up to the huge task alone, said the report by the UN human rights office.

It called on Colombo to remove from office military and security force personnel and any other officials "where there are reasonable grounds to believe that they were involved in human rights violations" in the 26-year war that ended in 2009.

Government soldiers on a beach inside the 'no fire zone' in the final battle against the LTTE. ( Reuters: Sri Lankan military handout/file )

The report named no suspects, saying it was a "human rights investigation, not a criminal investigation".

"This report is being presented in a new political context in Sri Lanka, which offers ground for hope," UN high commissioner for human rights Zeid Ra'ad Al-Hussein said in a statement.

"It is crucial that this historic opportunity for truly fundamental change is not allowed to slip."

The UN report, delayed from March to give the new government time to address concerns, found "patterns of grave violations" between 2002 and 2011.

It said Sri Lanka should set up a "hybrid special court integrating international judges, prosecutors, lawyers and investigators" to try war crimes and crimes against humanity allegedly committed by both sides.

"Ending the impunity enjoyed by the security forces and associated paramilitary groups, as well as holding to account surviving members of the LTTE [Liberation Tigers of Tamil Eelam], will require political will and concerted efforts," the two-part report of nearly 300 pages said.

No-fire zone violations

Government security forces are implicated in "unlawful killings carried out in a widespread manner against civilians" including ethnic minority Tamil politicians, aid workers and journalists, the report said.

The report said the security forces used brutal torture, including rape, especially when former LTTE members and civilians were detained after fighting ended.

"Not a single perpetrator of sexual violence related to the armed conflict is so far known to have been convicted," the report said.

Civilians escape an area controlled by Tamil Tigers in the Mullaittivu district in north-eastern Sri Lanka. ( AFP: Sri Lanka Defence Ministry/file )

Many attacks in the war's last phase — when the army surrounded the LTTE in a 300-square-kilometre slice of jungle in the northeast where 250,000 people were trapped — appear to have violated international law.

The report said violations were especially prominent with "repeated shelling by government forces of hospitals and humanitarian facilities in the densely populated no-fire zones" declared by the government.

"Witnesses gave harrowing descriptions of the carnage, bloodshed and psychological trauma of bombardments in which entire families were killed," it said.

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"The LTTE caused further distress by forcing adults and children to join their ranks and fight on the front lines."

The LTTE assassinated public officials and dissenting Tamil political figures and killed civilians in suicide bombings and mine attacks, the report said, adding that they had used child soldiers extensively, which is a war crime.

According to a UN panel report in 2011, up to 40,000 Tamils were killed in a final offensive ordered by then-president Mahinda Rajapaksa, who crushed the insurgency.

The latest report said it was "likely tens of thousands lost their lives" in the final stages.

The UN investigation, set up in March 2014, said there was a "systemic weakness in addressing these crimes especially when the military or security forces are involved".

Reuters