GENEVA — Nations have agreed to launch a U.N.-led investigation of Syria's bloody crackdown on its uprising, demanding that its government immediately stop the violence, release political prisoners and lift restrictions on the news media and access to the Internet.

In a 26-9 vote Friday that coincided with Syrian forces again opening fire on demonstrators, the U.N.'s top human rights body used a daylong special session to say it "unequivocally condemns the use of lethal violence against peaceful protesters by the Syrian authorities and the hindrance to access of medical treatment."

Human rights groups say about 500 people have been killed since the uprising began.

The Geneva-based Human Rights Council said it would ask the U.N. Office of the High Commissioner for Human Rights to urgently dispatch a mission to investigate "all alleged violations of international human rights law and to establish the facts and circumstances of such violations and of the crimes perpetrated."

The council said it wanted reports at its next full sessions and urged Syrian President Bashar Assad's government "to cooperate fully with and grant access to personnel from the mission" dispatched by the U.N. office. It also requested that U.N. Secretary-General Ban Ki-moon and the U.N.'s top human rights official, Navi Pillay, provide logistical support.

A simple majority vote was required to pass the resolution. China and Russia were among those opposed to it as political meddling. Saudi Arabia and six other nations abstained.

The U.N. nuclear agency, meanwhile, was setting the stage for more potential international action on Syria. Diplomats in Vienna said the agency will report that a Syrian target bombed by Israeli warplanes in 2007 probably was a secretly built nuclear reactor meant to produce plutonium. Syria denies the unfinished building had any such uses.

U.N. human rights deputy chief Kyung-wha Kang said the Syrian government "risks creating a downward spiral of anger, violence, killings and chaos" through tactics such as ordering tanks and other artillery to fire on peaceful pro-democracy protesters and snipers to shoot people trying to help the injured or remove dead bodies from public areas. She said around 1,800 people also have been injured in Syria.

"Any official ordering or undertaking of attacks against the civilian population can be held criminally accountable," she said. "Such attacks that occur on a widespread or systematic basis may amount to crimes against humanity."

However, Syria's U.N. ambassador, Faysal Khabbaz Hamoui, said the council was misguided since his nation was only defending itself against extremists.

"The council is acting under the pretext of humanitarian action to meddle in the internal affairs of a country," he told the council. "It's a return to a colonialist mentality."

Nigerian diplomat Ositadinma Anaedu, speaking for the African Union, cautioned that any council action could be interpreted as "political," and Chinese diplomat Xia Jingge warned the U.N. could further complicate the situation in Syria and undermine the council's own credibility.

Assad promised reforms last week and ditched the emergency laws the government has been using for a half-century to detain people. But critics point out he has continued to try to violently quell the protests that are the gravest challenge to his family's 40-year ruling dynasty.

"To the brave people of Syria, who are demanding freedom and dignity, we are here to say that the world stands by you, and we will not ignore your plight, said the U.S. ambassador to the council, Eileen Chamberlain Donahoe.

The United States and the European Union led the effort to send a strong message to Assad's authoritarian regime.

It was only the second such special session the council has ever called. The first such gathering was in late February to deal with Libya.

Angola, Bahrain, Jordan, Qatar were absent from the vote on Syria; Libya did not participate because its council membership was suspended.

Arab and African nations forced the council to remove from the U.S.-drafted resolution some contentious items, notably any criticism of Syria's uncontested candidacy for a seat on the council itself. A vote to elect new council members will be held next month in the U.N. General Assembly.

"Now that the Syrian government is under investigation by the Human Rights Council, electing Syria to the council would be like inviting the accused to sit in with the jury," said Philippe Bolopion, U.N. director of Human Rights Watch in New York. (AP)