The U.S. and Germany say the United Nations Security Council must respond to Syria's deadly crackdown on protesters after U.N. investigators detailed grave rights abuses they say were ordered by the “highest levels” of President Bashar al-Assad's government.

The U.S. Ambassador to the United Nations, Susan Rice, said Monday that with the Arab League's recent decision to impose sanctions on Syria and the “now well-documented atrocities” outlined in the U.N. report, it is “time to revisit” possible action by the Security Council.

A U.N. commission investigating allegations of human rights violations in Syria accused government troops of “summary execution; arbitrary arrest; enforced disappearance; torture, including sexual violence; as well as violations of children's rights.”

The report released Monday said Syrian forces have killed 256 children, including a 2-year-old girl, and that “torture was applied equally to adults and children.”

It cited numerous testimonies indicating that “boys were subjected to sexual torture in places of detention in front of adult men.”

The group's findings will now go to the Human Rights Council and the U.N. General Assembly, which will decide what to do next.

Meanwhile, the U.S. and the European Union Monday urged Syria to “end violence immediately” and allow a peaceful and democratic transition of government. A joint statement also called on Damascus to grant human rights observers and foreign journalists access to the country.

The EU said it plans to impose additional sanctions on Mr. Assad's embattled government. EU foreign ministers will vote Thursday on proposals to further restrict trade and economic dealings with Damascus.

Syrian Foreign Minister Walid Muallem said Monday the Arab League had declared “economic war” against Syria when it launched broad trade sanctions against it.

His remarks were the first official reaction to measures imposed by the Arab League on Sunday after Syria refused to accept a deadline allowing league observers to monitor the government's response to the eight-month uprising.

The United Nations says more than 3,500 people have been killed since March in connection with the uprising.