The first United Nations monitors have arrived in Syria, as part of a peace deal aimed at ending more than a year of violence.
A UN peacekeeping spokesman said the first six of 30 planned observers had landed in the capital, Damascus, and will begin work on Monday.
Syria's shaky ceasefire was increasingly under threat with fresh clashes erupting on Sunday.
The Syrian government vowed a crackdown on a wave of "terrorist attacks" and its forces shelled the city of Homs on Sunday.
The initial team of UN ceasefire monitors will be joined by two dozen more observers in coming days in line with a UN Security Council resolution adopted on Saturday.
The number of monitors is likely to rise to 250.
The Syrian government said it could not be responsible for the safety of the monitors unless it is involved in "all steps on the ground", government spokeswoman and presidential adviser Bouthaina Shaaban said.
She said Syria reserved the right to agree on the nationality of those participating.