The United States has warned that heightened violence in Syria threatens the viability of sending United Nations ceasefire monitors to the country.
The warning, from American ambassador to the UN Susan Rice, comes as an advance group of six monitors starts work in Damascus.
The UN is expected to approve a full mission later this week, the BBC reports. Eventually the mission is expected to number 250 observers.
Opposition activists say heavy shelling by government forces has continued in the city of Homs and 30 people were killed.
Under the peace plan put forward by UN-Arab League envoy Kofi Annan, the government in Damascus should have withdrawn its forces and heavy weaponry from cities on 10 April.