NEW YORK--The U.N. top human rights agency on Thursday issued a scathing account of Syria’s human rights abuses over the course of a bloody, four-month-long crackdown that has left nearly 2,000 protesters dead, and urged the U.N. Security Council to consider authorizing an investigation into crimes by the International Criminal Court prosecutor.

The U.N. Office of the High Commissioner for Human Rights accused the Syrian government of engaging in a “pattern of human rights violations that constitutes widespread or systematic attacks against the civilian population, which may amount to crimes against humanity.”

The release of the 22-page report came on a day in which President Obama and the leaders of Britain, France and Germany issued calls for Syrian President Bashar al-Assad to relinquish power. It came hours before the U.N. High Commissioner for Human Rights, Navi Pillay, was due to brief the U.N. Security Council on her findings.

The council’s four European powers, led by Britain and France, with the backing of the United States, have been struggling to convince the 15-nation Security Council to take a tougher line on Syria. But they have faced resistance from Brazil, China, India, Russia and South Africa, who maintain that the Syrian leader should be given more time to demonstrate he is committed to implementing sweeping political reforms.

But pressure has been mounting on Syria. The U.N. Human Rights Council scheduled a meeting on Monday to review Syria’s human rights record. Four Gulf countries, Qatar, Kuwait, Jordan and Saudi Arabia, joined the European initiative to convene the meeting.

The high commissioner’s report, which covers the period from March 15 to July 15 and is based in part on interviews with 180 Syrian refugees, asserts that several security agencies have responded to Syria’s popular uprising with “an apparent shoot-to-kill policy.”

The U.N. rights agency has compiled a list of the names of 1,900 civilians killed in the crackdown, including 353 civilians who were summarily executed. The agency has also assembled a second confidential list of 50 potential perpetrators whose names could be passed on to a future international prosecutor.

Survivors endured severe hardship, including psychological and physical torture, unlawful arrest, deprivation of basic services, including water and medicine, and “routine humiliation,” according to the report. The “victims of arbitrary arrests declared they were beaten and humiliated with insults referring to their religious, democratic and political beliefs. “

The report documents a chilling catalogue of alleged state crimes as Syrian security forces, joined by ethnic Alawite militia, known as the Shabbiha, laid siege to restive town after town, shelling civilians with tank, artillery and helicopter fire, and picking off unarmed civilians, including children, with sniper fire as they left their homes in search of food. Syrian soldiers who refused order to kill civilians were themselves executed, according to the accounts.

The report challenges the Syrian government’s claim that it is confronting an armed opposition to its rule, saying that while some civilians took up arms, the vast majority of killings were carried out by Syrian security forces against unarmed civilians. The Syrian government refused repeated request by the high commissioner’s office to travel to Syria to verify the claims of abuses.

A day before the report’s release, Assad promised U.N. Secretary General Ban Ki-moon that the military police operations in Syria had been called off, and he offered to accept a U.N. team to conduct a humanitarian assessment of the situation in Syria. Assad made a previous pledge to Ban in May but never honored it.

Thursday’s report claims that the uprising has its roots in the town of Dar’a, following the arrest of a “group of youths and children” on charges of writing anti-government graffiti. In mid-March, relatives of the detainees appealed to Syrian officials for their release, “but they were both rebuffed and insulted.”

In response, a demonstration was organized on March 18, following Friday prayers at a local Mosque. The Syrian authorities responded with tear gas and firing of live ammunition in an operation that killed four civilians and unleashed a chain of events that spread unrest across the country.

“Whilst there have been violent incidents caused by a minority of civilians in some demonstration…reports from a variety of sources assert that the demonstrations were mostly peaceful,” according to the report. “The majority of killings reported were due to live ammunition, coming from security forces, the military and Shabbiha elements, using Kalashnikovs and other guns.”

In one typical operation, Syrian security forces opened fire, without warning, on a group of peaceful demonstrators brandishing olive branches in the village of al-Mastuma, south of Idlib. “Some of the estimated 20 security personnel had hidden behind some trees while others were positioned on rooftops. An estimated 200 people were injured and 30 others killed, some of them reportedly ‘being finished of with knives as they lay on the ground.’”

Victims of state violence in other towns needlessly succumbed to their wounds because Syrian security forces had either shut down local hospitals or prevented them from treating wounded demonstrators.

“Due to lack of access to Syria, the [UN] Mission was unable to verify repeated allegations that civilians were routinely and summarily executed in their hospital (or make-shift hospital) beds by security forces. However, it was widely reported that forces conducted regular raids in hospitals to search for and kill injured demonstrators.” Snipers also targeted people providing medical assistance to victims.

The report cites numerous cases in which Syrian forces defected in response to instructions to kill unarmed civilians. Those who were caught paid the ultimate price. “They stated that they received clear orders to use live ammunition against protestors,” the U.N. report stated. “Those who did not shoot civilians were shot from behind by other security forces and Shabbiha units. “