GENEVA (AP) -- The killings took place under a late spring sun. The squads poured into the Syrian village of Houla on foot and in vehicles: cars, minivans and pickups mounted with machine guns.

At the end, more than 100 people were dead -- nearly half of them children -- in what the United Nations described for the first time Wednesday as a war crime perpetrated by the government forces and shabiha militia backing the regime of President Bashar Assad.

People who tried to remove the bodies were shot at. No one who'd survived the massacre had fled to a nearby national hospital that was accessible on foot -- they knew the army had occupied it for months. And when U.N. observers arrived the next day to check on the scene, they found government troops in control.

In a highly anticipated report that spells out clear responsibility for attacks on civilians, an independent commission appointed by the U.N. Human Rights Council laid the groundwork for possible prosecutions in international courts against the Syrian leader and other senior government officials.

The panel's report also contained an ominous warning that Syria's civil war was moving in "brutal" directions on all fronts as Assad's forces step up air assaults and anti-government armed groups seek stronger firepower to fight back.

It found that the regime and pro-government shabiha militia were directly responsible for the killing of more than 100 civilians in Houla in late May. The Syrian government had reported that the Syrian Army was defending itself at Houla from an attack by "terrorists," and said some of its soldiers were killed in the clashes.

But the U.N. panel's report to the 47-nation Human Rights Council, which is based in Geneva, says it determined that the Houla killings and numerous other murders, unlawful killings, acts of torture, rape and other sexual violence and indiscriminate attacks on civilians were carried out "pursuant to state policy pointing to the involvement at the highest levels of the armed and security forces and the government."

According to the final 102-page report to the council, in one village site where around 60 people were killed, the commission found through satellite imagery and corroborated accounts that "the movement of vehicles or weapons, as well as the size of the group, would have been easily detectable by government forces," but the place was inaccessible for any "sizeable" anti-government armed group.

It says 40 separate interviews were all "consistent in their portrayal of the events and their description of the perpetrators as government forces and shabiha."

The panel also concluded that anti-government armed groups committed war crimes, including murder, extrajudicial killings and torture, but said that "these violations and abuses were not of the same gravity, frequency and scale" as those carried out by government forces and the shabiha militia.

A confidential list of people and armed units believed to be responsible for crimes against humanity, breaches of international humanitarian law and gross human rights violations will be submitted to the U.N. High Commissioner for Human Rights Navi Pillay in September.

In its use of the term "war crimes" to describe its findings, the panel relies on an assessment of Syria by the International Committee of the Red Cross in mid-July. The Geneva-based ICRC, which oversees the Geneva Conventions known as the rules of war, said it now considers the 17-month-old conflict in Syria to be a full-blown civil war, meaning international humanitarian law applies throughout the country.

That assessment was an important reference for the panel and for all others trying to determine how much and what type of force can be used. It forms the basis for war crimes prosecutions, especially if civilians are attacked or detained enemies are abused or killed. Until then, everything had to be considered "a violation or an abuse," said Karen Koning AbuZayd, one of the panel members.

"The international community must come to some kind of consensus to stop the violence in the first place and then eventually hold people accountable for it," AbuZayd, a U.S. citizen and former head of UNRWA, the U.N. agency that aids Palestinian refugees, told The Associated Press.

Syria had previously won her respect, she said, because of the excellent care it provided to Palestinian refugees during her past decade of work there.

"For me, everything is pretty shocking. For these things to be happening in Syria ... It's pretty painful, I must say, to see what is happening to the country and how all sides are behaving, really," she added.

The panel's report was issued just hours after a bomb exploded in Syria's capital, Damascus, outside a hotel where U.N. observers are staying. The bomb was attached to a fuel truck and wounded at least three people, Syrian state TV reported. Activists also reported fighting near the government headquarters and the Iranian Embassy, both in Damascus, along with clashes in different parts of Syria.

The panel was appointed to probe abuses in Syria, but had hardly any access to the country, with only its chairman, Brazilian diplomat and professor Paulo Sergio Pinheiro, allowed into Damascus for a weekend visit last month to meet with some top government officials and families affected by the violence. A third panel member had dropped out.

Most of the report, which covers the period between Feb. 15 and July 20, was conducted during field interviews and in Geneva with Syrian refugees outside the country. It is based on 1,062 interviews, but the panel emphasized that the investigation was hampered by the Assad regime's unwillingness to cooperate.

Their report, whose findings are strikingly more conclusive about the Houla massacre than previous interim findings, could be used by world powers to justify tougher outside action against Syria, or strengthen calls for an international investigation and prosecution of possible war crimes and crimes against humanity.

The panel recommends that the president of the Human Rights Council forward the report to U.N. Secretary-General Ban Ki-moon, who could bring it to the attention of the most powerful arm of the U.N., the Security Council based in New York. Earlier this year, the council said in a resolution that it agreed with Pillay, the U.N.'s top human rights official, in her call for action by the International Criminal Court based at The Hague.

But Russia and China, two of the five veto-wielding permanent members on that 15-nation council, have effectively blocked major powers from responding in a coordinated fashion to the Syria crisis, a stalemate cited by the U.N.-Arab League special envoy to Syria, Kofi Annan, in deciding to resign from his post at the end of this month.

The Human Rights Council also could extend the mandate of its special panel on Syria, so that it can investigate further. But even if it doesn't, Pinheiro would continue to work in essentially the same role for the council as its special rapporteur for Syria, a position that was created in March.

Activists say more than 20,000 people have been killed since the start of Syria's revolt, inspired by other Arab Spring uprisings against autocratic regimes in the region. The conflict has slowly changed into a full blown civil war that the panel says involves "more brutal tactics and new military capabilities on both sides."