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All sides in Syria's civil war are using shelling and siege tactics to punish and starve civilians and big powers bear responsibility for allowing such war crimes to persist, U.N. human rights investigators said on Wednesday.

The independent investigators, presenting their latest report documenting atrocities in Syria, called again on the U.N. Security Council to refer grave violations of the rules of war to the International Criminal Court (ICC) for prosecution.

"The Security Council bears responsibility for not addressing accountability and allowing the warring parties to violate these rules with total impunity," Paulo Pinheiro, who leads the U.N. commission of inquiry, told a news conference.

"One of most stark trends we have documented is the use of siege warfare, the denial of humanitarian aid, food and basic necessities such as medical care and clean water have forced people to choose between surrender and starvation."

More than 140,000 have been killed in the conflict, which enters its fourth year next week, 2.5 million refugees have fled abroad and 6.5 million people are uprooted within Syria.

Syrian government forces under President Bashar al-Assad have besieged towns including the Old City of Homs, shelling relentlessly and depriving them of food as part of a "starvation until submission" campaign, the report said.

On the other side, insurgents fighting to topple Assad, especially foreign Islamic fighters including the al-Qaeda affiliated ISIS, have stepped up attacks on civilians, taken hostages, executed prisoners and set off car bombs to spread terror, it said.

The report, covering July 15-January 20, is the seventh by the United Nations since the inquiry was set up in September 2011.

The investigators have not been allowed into Syria, but their latest findings were based on 563 interviews conducted by Skype or by telephone with victims and witnesses still in the country or in person with refugees in surrounding countries.

- Reuters