A United Nations probe has found that Syrian troops killed hundreds of children and committed other "crimes against humanity" since the government crackdown began in March.

A panel of independent experts says at least 256 children were killed by government forces as of early November, with some boys sexually tortured and a two-year-old girl shot to death just to prevent her from growing up to be a demonstrator.

The panel's report to the UN Human Rights Council says government forces have "shot indiscriminately at unarmed protesters" while snipers targeted others in the upper body and head.

Their report, released Monday, said Syrian security forces along with militias were given "shoot to kill" orders to crush demonstrations.

U.S. Ambassador to the UN Susan Rice issued a statement welcoming the report for documenting what she described as the Assad regime's "ruthless, depraved campaign against its own people."

"The details are appalling," Rice said. "Among the commission's findings are evidence of systemic government support for the rape and torture of children. The world can now clearly see what boundaries the Assad regime is willing to cross to retain its grip on power."

In Ottawa, Foreign Affairs Minister Minister John Baird called the report "deeply disturbing."

"Obviously it's further evidence the Assad regime should go immediately," he told reporters.

Syria calls sanctions 'economic war'

Meanwhile, Syria's foreign minister says the Arab League's newly approved sanctions against Syria amount to "a declaration of economic war."

Foreign Minister Walid al-Moallem's remarks at a televised appearance in Damascus on Monday come one day after the Arab League approved sweeping sanctions targeting Syria for its crackdown on an eight-month-old uprising. The United Nations says more than 3,500 people have been killed.

P.O.V. Do you think the Arab League's sanctions on Syria will be effective? Take our survey.

Al-Moallem told reporters that the Arab League and others refuse to believe that there is a foreign conspiracy against Syria.

The sanctions by Syria's Arab neighbors are expected to squeeze an ailing economy that already is under sanctions by the U.S. and the European Union.

Economy Minister Mohammed Nidal al-Shaar, in remarks published Monday in the Syrian pro-government daily Al-Watan, said the sanctions are a "political decision and a dangerous precedent that would eventually have a bad impact on Syrian citizens."

Once they take force, he said, "sources of foreign currency would be affected." The comment reflected concern that Arab investment in Syria will fall off and transfers from Syrians living in other Arab nations will also drop.

The sanctions are among the clearest signs yet of Syria's growing international isolation. Damascus has long boasted of being a powerhouse of Arab nationalism, but Assad has been abandoned by some of his closest allies and now his Arab neighbours.

Massive protests against sanctions

Still tens of thousands of government supporters flocked to main squares in almost all cities, including the capital Damascus, to denounce the Arab League decision. State-TV quoted people as saying that the sanctions target all segments of the population.

The European Union and the United States already have imposed sanctions, the League has suspended Syria's membership and world leaders increasingly are calling on Assad to go.

But as the crisis drags on, the violence appears to be spiralling out of control as attacks by army defectors increase and some protesters take up arms to protect themselves.

Arab League Secretary General Nabil Elaraby said the bloc will reconsider the sanctions if Syria carries out an Arab-brokered plan that calls for pulling tanks from the streets and ending violence against civilians.

The regime, however, has shown no signs of easing its crackdown, and activist groups said more than 30 people were killed on Sunday alone. The death tolls are impossible to confirm independently because Syria has banned most foreign journalists.

At a news conference in Cairo Sunday, Qatari Foreign Minister Hamad bin Jassim said 19 of the League's 22 member nations approved a series of tough sanctions that include cutting off transactions with the Syrian central bank, halting Arab government funding for projects in Syria and freezing government assets. The sanctions take effect immediately.

Iraq and Lebanon — important trading partners for Syria — abstained from the vote, which came after Damascus missed an Arab League deadline to agree to allow hundreds of observers into the country as part of a peace deal Syria agreed to early this month to end the crisis.