Syrian forces continued to attack rebel strongholds Tuesday, with activist groups saying mortar shells hit anti-government protesters in the eastern town of Deir al-Zour, killing 10 people as fighting raged elsewhere.

The attack on the town close to the Iraqi border came hours after a car bomb killed a dozen people there.

In the western town of al-Haffeh, activists say hundreds of rebels are facing a continued tank and helicopter-backed assault. The helicopter attacks – confirmed by the U.N. on Monday – are regarded as a significant escalation by government forces.

United Nations spokeswoman Sausan Ghosheh said Tuesday that U.N. monitors who had travelled towards al-Haffeh to investigate reports of clashes in the area found it too dangerous to enter. The United States fears Syrian forces are planning to massacre civilians in the town

Clashes began last week when security forces initiated their attempt to capture the strategic Sunni Muslim town, located close to the port city of Latakia and the Turkish border – and used by rebels as an active smuggling route for people and supplies.

The spokesman for international envoy Kofi Annan said Tuesday that an international “contact group” will meet soon to discuss how to pressure Syria's government and opposition groups to implement the U.N.-Arab League envoy's tattered peace plan.

The contact group meeting has been in doubt because of western opposition to Syrian ally Iran's involvement. The Iranian foreign ministry Tuesday welcomed a Russian proposal for Iran to be included despite strong reservations from the U.S., France and Britain.

On Monday, the United Nations accused Syrian security forces and pro-government shabiha militias of committing serious rights violations against children, including using them as human shields. The report also said children have been “victims of killing and maiming, arbitrary arrest, detention, torture and ill-treatment, including sexual violence.”

The U.S.-based Human Rights Watch said the U.N. Security Council should impose an arms embargo and other targeted sanctions on the Syrian leadership in response to the abuses described in the report. It said Syrian children are paying a “horrendous price” in the conflict.

The Britain-based Syrian Observatory for Human Rights said nationwide attacks by government and rebel forces killed at least 63 civilians and 21 troops on Monday.

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