All sides in Syria's civil war are using shelling and siege tactics to punish civilians.

UN human rights investigators said big powers bear responsibility for allowing such war crimes to persist.

In their latest report documenting atrocities in Syria, the investigators called again on the UN Security Council to refer grave violations of the rules of war to the International Criminal Court for prosecution.

"The Security Council bears responsibility for allowing the warring parties to violate these rules with impunity," the report by the UN commission of inquiry on Syria said.

"Such inaction has provided the space for the proliferation of actors in the Syrian Arab Republic, each pursuing its own agenda and contributing to the radicalisation and escalation of violence."

Divided world powers have supported both sides in Syria's three-year conflict and a diplomatic deadlock has exacerbated the bloodshed.

The independent investigators, led by Brazilian expert Paulo Pinheiro, said that fighters and their commanders may be held accountable for crimes, but also states that transfer weapons to 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.

It said the Syrian air force had dropped barrel bombs on Aleppo with "shocking intensity", killing hundreds of civilians and injuring many more.

Insurgents fighting to topple President 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 15 July 2013 to 20 January 2014, is the seventh by the United Nations since the inquiry was set up in September 2011, six months after the anti-Assad revolt began.

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.

All sides have violated the rules of war embodied in the Geneva Conventions, according to the team of two dozen, which includes former UN war crimes prosecutor Carla del Ponte.

It has now drawn up four confidential lists of suspects.

Despite some tactical gains by Syrian government forces backed by more foreign combat forces of Lebanese Hezbollah and Iraqi militia, the fighting has reached a stalemate, causing significant casualties and material losses, the report said.

It said: "The government relied extensively on the superior firepower of its air force and artillery, while non-state armed groups increasingly resorted to methods of asymmetric warfare, such as suicide bombs and use of improved explosive devices."

It said government forces have besieged and bombarded civilian areas as part of a strategy aimed at weakening the insurgents and breaking the will of their popular base.

"Partial sieges aimed at expelling armed groups turned into tight blockades that prevented the delivery of basic supplies, including food and medicine, as part of a 'starvation until submission' campaign."

Rebels throughout Syria have "inflicted severe physical or mental pain or suffering on civilian populations in areas under their control", including on prisoners, it said.