The United States and Germany on Monday led Western calls for the divided UN Security Council to act on Syria's deadly assault against protests after UN investigators said crimes against humanity had been committed.

"It is past time for the Security Council to take much more decisive action with respect to Syria," said US ambassador Susan Rice. The Council cannot "stand idly by," added Germany's UN envoy Peter Wittig.

The 15-member council was split last month by a European-drafted resolution condemning President Bashar al-Assad's crackdown. Russia and China vetoed the resolution, while Brazil, India, South Africa and Lebanon abstained.

But a report by a UN human rights commission, which said crimes against humanity, had been ordered by the "highest levels" of Assad's government, and the Arab League decision to order sanctions have strengthened the calls for action.

The United States and its European allies have already condemned the rare double veto by Russia and China.

But Rice said that with the Arab League sanctions and the "now well-documented atrocities" outlined in the UN report "we think it is time to revisit the question of what might be possible here in New York."

Wittig called the Arab League sanctions "historic". "The council here cannot stand idly by regarding what the regional organization has said so strongly. We think the council should take up that decision and endorse it and reinforce it," he told reporters. Rice and Wittig said informal talks on possible action would soon start.

Because of the internal divisions, the Security Council has so far only agreed a statement, with less moral weight, on the violence. The 193 member UN General Assembly passed a resolution last week deploring the violence.

And rights groups also stepped up demands for Security Council action following the new UN report on killings and arrests in Syria since mid-March. The UN says more than 3,500 people have been killed.

Amnesty International called on the Security Council to refer the case to the International Criminal Court, order an arms embargo and freeze the assets of Assad and his associates.

"Continued inaction by the Security Council will not only allow the commission with impunity of more human rights violations in Syria, but embolden present and future violators," said an Amnesty statement.

Human Rights Watch said the UN Human Rights Council must refer the Syria case to the UN Security Council and call for it "to impose targeted sanctions and refer the situation in Syria to the International Criminal Court."  -AFP