Experts commissioned by the UN say army officers and government officials are responsible for human rights violations.
Syrian forces have renewed shelling of the opposition stronghold of Homs.
On Wednesday two Western journalists were killed by shelling in the most heavily besieged district of the city.
There is growing pressure on Damascus to give access to civilians trapped by the onslaught.
The UN had asked a panel of experts to report on the situation in Syria, in a follow up to an investigation published in November.
The UN secretary general says he wants to send his humanitarian chief to Syria to negotiate access to Homs.
Dozens of people were killed in Syria on Wednesday, including journalists Marie Colvin and Remi Ochlik.
Their deaths brought further condemnation of the Syrian government, which has denied responsibility for the killings.
Aid workers trying to retrieve the journalists' bodies say they may have to be buried in Syria, as the roads out of Homs are blocked by the Syrian army.
On Thursday, rockets, artillery and mortar rounds pounded the opposition stronghold of Baba Amr, Inshaat and Khalidiya, activists said.
Activist Hadi Abdullah told AFP that he could hear "terrifying explosions".
French Foreign Minister Alain Juppe said a ceasefire was "imperative" to enable food and medical supplies to be brought into Homs, which has been under siege since 4 February.
The city is one of the focal points of the uprising against the government of President Bashar al-Assad.
UK Foreign Secretary William Hague said the international community will be looking at further ways to apply pressure to Mr Assad.
"It is a deeply frustrating situation that people have been dying in their thousands... that the Assad regime has continued to act seemingly with impunity - but I think we can agree to a wider set of measures across a large group of nations," Mr Hague said.
The European Union sanctions on Syria could be tightened further, Mr Hague added.
Marie Colvin of the Sunday Times and French photographer Remi Ochlik were killed on Wednesday morning in the house in which they were staying in Baba Amr.
Marie Colvin and Remi Ochlik were killed in a makeshift media centre in Homs
Three other Western journalists were wounded in the strike on the house, among them a British photographer, Paul Conroy, whose injuries are not thought to be life-threatening, and the French reporter Edith Bouvier. Her situation is believed to be more serious.
France and the UK have demanded that they be given urgent medical assistance.
The editor of the Sunday Times, John Witherow, said he thought it was possible that the journalists had been deliberately targeted, speculating that government forces may have been able to use technology to identify their precise location.
"It seems to me perfectly reasonable to assume that they would have targeted them," he told the BBC.
World and Arab leaders are due to meet in Tunisia on Friday to discuss Syria's future.
But Russia, a key ally of President Assad, says it will not attend the "Friends of Syria" meeting because the Syrian government would not be represented.
Russia and China have faced Western and Arab criticism for blocking a UN Security Council resolution that would have backed an Arab League peace plan for Syria.
Opposition groups claim more than 7,500 people have been killed since the start of the uprising 11 months ago.
The Syrian government says at least 2,000 of its forces have died fighting "armed terrorists," as it calls the rebels.
Source: BBC News