Syria has failed to meet its obligations under a UN-backed peace plan to withdraw troops and heavy weapons from residential areas, the UN Secretary General said on Wednesday.
"The Syrian Government has yet to fully implement its initial obligations regarding the actions and deployments of its troops, or to return them to barracks," Ban Ki-moon was quoted by al Arabiya as saying.
In a letter to the UN Security Council, Ban called for an expanded 300-strong observer mission, saying there was an "opportunity for progress." Ban said he wanted monitors to be deployed to ten locations in the coming weeks.
An advance team of six observers enjoyed "freedom of movement" during their visit the southern city of Deraa on Tuesday, Ban said, but were barred from going to the restive city of Homs because of "security concerns."
Ban said violence has continued in Syria despite a truce agreed a week ago following a UN Security Council resolution, and "the cessation of armed violence in all its forms is therefore clearly incomplete."
Both sides said they were committed to ending the "violence in all its forms," he added.
Government troops shelled four districts of the battered city of Homs on Wednesday, the British-based Syrian Observatory for Human Rights said. The state-run SANA news agency said six members of the security forces were killed in a "terrorist" bombing in the northern province of Idlib.
Syrian Foreign Minister Walid al-Muallem vowed on Wednesday to "respect and implement" the peace plan which was brokered by UN and Arab League envoy Kofi Annan, and said he agreed to the UN deplying 250 observers on the ground in Syria.
One Russian observer has already arrived in Syria, and three others are poised to go, Russian Deputy Foreign Minister Mikhail Bogdanov told reporters in Moscow.
He said the exact number of Russian observers is yet to be determined.
The UN Security Council is due to meet later on Thursday to discuss Annan's report on the latest developments in Syria.
The Friends of Syria group is also meeting in Paris later today.