The head of the UN observer mission to Syria has warned that even 1,000 unarmed observers could not end the violence in the country on their own.
Arriving in Damascus, Maj Gen Robert Mood urged all sides to cease fighting and said peace must be a joint effort.
He will be followed by another 30 observers in the coming days, doubling the size of the mission. The UN has approved up to 300 observers.
Activists say at least 25 people were killed across the country on Sunday.
At least 500 have died since the ceasefire was agreed on 12 April, they say.
The government and opposition have blamed each other for the violence.
Activists say shelling continued in several provinces, while the government blamed armed terrorist groups for a series of attacks around the country.
There were also reports that two people were killed by snipers in the city of Homs. Violence there has fallen after two UN observers were permanently stationed there two weeks ago.
So far there are around 30 unarmed observers in Syria, and Maj Gen Mood said that would double in the coming days as part of a rapid buildup to the 300 the UN agreed would be deployed.
Plea for support
"Ten unarmed observers, 30 unarmed observers, 300 unarmed observers, even a thousand unarmed observers cannot solve all the problems," Maj Gen Mood told reporters as he arrived at Damascus airport.
"To achieve the success of Kofi Annan's six-point plan, I call on all to stop the violence and to help us on a continued cessation of armed violence in all its forms," he added.
Officials in Lebanon are continuing to question the crew of a Sierra Leone-registered ship which was found to be carrying a large consignment of weapons which it is thought were destined for use by Syrian rebels.
Maj Gen Mood said the ceasefire could only work with the support of the forces on the ground. "It's absolutely necessary to understand that this a joint effort," he said.
"The most important element is the Syrians themselves, the rights, the aspirations of the Syrian people. Then we have the observers on the ground - unarmed, representing the will of the international community, the will of the UN to be there with them. Then you have the political element between states in the region, and we have the diplomatic level on top.
"To reach a positive way forward, and we have that choice, all these elements have to play together."
On Thursday UN Secretary General Ban Ki-moon warned that Syrian's government was "in contravention" of a UN- and Arab League-backed peace plan.
Mr Ban has demanded that Damascus comply with the peace plan brokered by international peace envoy Kofi Annan without delay.