(Adds Annan spokesman quotes, background, details)
By Marwan Makdesi and Tom Miles
DAMASCUS/GENEVA, April 19 (Reuters) - Syria and the United Nations signed an agreement on Thursday on the terms of a ceasefire monitoring mission, the Syrian Foreign Ministry said.
"This preliminary agreement ... aims to facilitate the task of the observers within the framework of Syrian sovereignty," the statement said.
A statement from U.N.-Arab League mediator Kofi Annan said the Syrian government and the United Nations had agreed a basis for a "protocol" on the deployment of more observers.
"This agreement outlines the functions of the observers as they fulfil their mandate in Syria and the tasks and responsibilities of the Syrian government," Ahmad Fawzi, Annan's spokesman, said in a statement.
Fawzi said Annan's team was holding "similar discussions with representatives of the opposition on the tasks and responsibilities of the armed opposition groups."
The 15-member U.N. Security Council will meet at 9:00 a.m (1300 GMT) on Thursday for a briefing by Annan's deputy, Jean-Marie Guehenno, which will be crucial in determining whether the conditions are right for deploying a larger monitoring mission to Syria.
U.S. and European diplomats on the council have suggested that Syria's lack of full compliance with its obligations to end the violence may make it difficult for them to support a new resolution that would be needed to deploy an expanded observer mission.
In a letter to the Security Council, U.N. chief Ban Ki-moon said Syria has not fully withdrawn troops and heavy weapons from towns, failing to send a "clear signal" about its commitment to peace, underlining Western fears for the week-old truce.
In the first progress report since the Security Council passed a resolution on Saturday authorising the deployment of observers, Secretary-General Ban proposed an expanded mission of 300 personnel to monitor a shaky ceasefire between forces loyal to President Bashar al-Assad and opposition fighters.
Ban said this week that 250 observers were too few in a country of 23 million where the United Nations says at least 9,000 people have been killed in the past 13 months. He sought European help in supplying planes and helicopters.
But Syrian Foreign Minister Walid al-Moualem said 250 was a "reasonable number", adding they should be from countries such as China, Russia, Brazil, India and South Africa - all more sympathetic to Damascus than are the West or the Arab League.
He also dismissed any need for U.N. aircraft. (Writing by Oliver Holmes; Editing by Tim Pearce)