Activists say fresh violence by Syrian security forces has killed at least 13 people in the city of Homs.

The British-based Syrian Observatory for Human Rights says the deaths occurred Monday when government forces shelled the neighborhood of Baba Amr.

The violence comes a day before Arab League monitors are scheduled to visit the city.

A group of about 50 observers is due to arrive in Syria Monday, to monitor compliance with a League plan that requires the government to end its suppression of a nine-month pro-democracy uprising.

Syria agreed to the observer mission under Arab League pressure.

On Sunday, Syria's top opposition leader called on the Arab League to seek backing from the United Nations. Paris-based Syrian National Council leader Burhan Ghalioun said the League should step up pressure on Syrian President Bashar al-Assad by asking the U.N. Security Council to adopt the Arab initiative.

Russia and China have so far resisted efforts to have the Security Council condemn the Syrian government and impose sanctions on it for continuing the crackdown.

The Syrian Observatory for Human Rights told VOA that pro-Assad forces killed at least six people in Homs on Sunday in shelling and shooting attacks. It said government forces also killed three people in the northeastern town of Deir Ezzor.

London-based Observatory spokesman Mousab Azzawi said Syrian military helicopters were flying over Homs to try to locate the signals of satellite phones used by local activists to inform the rights group about the crackdown.

It is not possible to independently verify casualty figures in Syria because the government bars international journalists from operating freely in the country.

The United Nations estimates 5,000 people have been killed in violence linked to the uprising since it began in March with protests against Mr. Assad's 11-year autocratic rule. Syria says “armed terrorists” are driving the revolt and accuses them of killing 2,000 security personnel during that period.