By msnbc.com staff and news services

Syrian authorities are detaining and torturing children, the United Nations' human rights chief, Navi Pillay said, according to a report.

"They've gone for the children -- for whatever purposes -- in large numbers," the BBC quoted her as saying. "Hundreds detained and tortured... it's just horrendous.

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"Children shot in the knees, held together with adults in really inhumane conditions, denied medical treatment for their injuries, either held as hostages or as sources of information."

Ms Pillay, a lawyer, said she believed that the UN Security Council had enough reliable information to warrant referring Syria to the International Criminal Court (ICC).

"I feel that investigation and prosecution is a crucial element to deter and call a stop to these violations," she told the BBC.

Ms Pillay said she believed that the UN Security Council had enough reliable information to warrant referring Syria to the International Criminal Court (ICC).

Syria accepts Annan peace plan, but clashes continue

Meanwhile, the United States has urged the Syrian opposition to unite and pledge to respect minority rights in a future Syria should President Bashar Assad be driven from power, and warned armed rebels and government forces against committing human rights abuses.

Disunity among the Syrian opposition to Assad has fed fears that Syria could slide into sectarian and ethnic conflict, much as Iraq did after the 2003 U.S.-led invasion that toppled Saddam Hussein.

Skeptical of peace plan
This has worried some governments, including the United States which would otherwise be glad to see Assad's downfall, after a year in which Assad has been using the army to crush efforts to end his political dominance in Syria.

Slideshow: Struggle in Syria

Str / AP

Anti-government clashes continue as Western and Arab nations launch a diplomatic offensive to halt the violence.

Launch slideshow

U.S. Secretary of State Hillary Clinton urged the Syrian opposition to lay out a vision of an inclusive Syria in which minority rights are respected.

"They must be able to clearly demonstrate a commitment to including all Syrians and protecting the rights of all Syrians," Clinton told reporters.

"We are going to be pushing them very hard to present such a vision in Istanbul," she said ahead of a gathering of Western and Arab nations in Istanbul on Sunday to discuss a political transition in Syria.

Earlier on Tuesday, the New York Times reported that a meeting of Syrian opposition groups in Istanbul was marred when a veteran dissident and Kurdish delegates walked out, saying their views were not heard.

U.S. Ambassador to Syria Robert Ford said on Tuesday in Washington that he had received reports that armed Syrian opposition groups had engaged in human rights abuses. He said he had warned the rebels, as well as Assad, against committing such abuses.

Both Clinton and Ford were skeptical of reports that Syria's government had accepted the peace plan of U.N.-Arab League envoy Kofi Annan.

"Given Assad's history of over-promising and under-delivering, that commitment must now be matched by immediate actions," Clinton said.

Ford left Syria last month because of the violence but remains the U.S. ambassador. At a hearing on Capitol Hill, he was asked about statements by the U.S.-based group Human Rights Watch that armed opposition groups in Syria had committed abuses including kidnapping, detention and torture of security force members and government supporters.

Murad Sezer / Reuters

Syrian National Council President Burhan Ghalioun is greeted by council members during a news conference after their meeting in Istanbul on Tuesday.

"We had reports like that last year, when some of the fighting in Homs became really serious," Ford said. "We raised it even in Syria when my embassy was still open.

"We discussed it with some of the local revolution council representatives -- who are themselves not members of armed groups, but certainly are in contact with them -- and emphasized that they would be held to a standard on this if they wanted support from western countries."

The United States had also raised the matter with the Syrian National Council, the main opposition umbrella group, Ford said.

He added there was a danger that more hard-liners who ignored human rights would gain influence on both sides in Syria the longer the conflict goes on.

Assad's government, Ford said, had committed "massive human rights violations that may amount to crimes against humanity."

The United Nations says more than 9,000 people have been killed in Syria's year-old uprising against Assad. Syria says rebels have killed some 3,000 security force members and blames the violence on "terrorist" gangs.

Human Rights Watch also has accused Assad's forces of human rights abuses, including using human shields in northern Syria in their efforts to crush the rebellion.

Assad on Tuesday was filmed taking a tour of Baba Amr, the district of Homs recently bombarded by his forces.

Reuters and msnbc.com staff contributed to this report.