A first team of Arab League monitors landed in Syria late Monday to monitor a deal to end nine months of deadly violence, as 30 people died in gunfire in and near the besieged central city of Homs.
The mission is part of an Arab plan endorsed by Syria on November 2 that calls for the withdrawal of security forces from towns and residential districts, a halt to violence against civilians and the release of detainees.
Since signing the deal, President Bashar al-Assad's regime has been accused of intensifying its crackdown, which has showed no signs of abating since it erupted in March and which the UN estimates has killed more than 5,000 people.
The private Dunya television channel, which is close to Assad's regime, said: "A delegation of 50 observers arrived on Monday evening in Damascus", adding that 10 team members were Egyptian.
General Mohammed Ahmed Mustafa al-Dabi, a veteran Sudanese military intelligence officer who is heading the Arab observer mission, arrived in the Syrian capital on Sunday.
Syria's opposition meanwhile urged United Nations and Arab League intervention as the spiral of violence continued.
Syrian National Council (SNC) head Burhan Ghaliun told reporters in Paris that some observers were already in Homs "but they are saying they cannot go where the authorities do not want them to go."
He said he was seeking UN and Arab League intervention "to put an end to this tragedy," and urged the UN Security Council to "adopt the Arab League's plan and ensure that it is applied."
"It is better if the UN Security Council takes this (Arab League) plan, adopts and provides the means for its application," Ghaliun said. "That would give it more force."
The Arab "plan to defuse the crisis is a good plan, but I do not believe the Arab League really has the means" to enforce it, he said.
"The observers are working in conditions that the Arab League has described as not being good. ... I think we have not properly negotiated the working conditions of the observers," Ghaliun added.
The Britain-based Syrian Observatory for Human Rights said "rocket fire and heavy machineguns in the Baba Amro quarter" of Homs killed 18 people.
"The situation is frightening and the shelling is the most intense of the past three days," it said in a statement received by AFP in Nicosia.
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Eleven civilians died in other parts of Homs and its suburbs, and a woman was killed at Talbisseh near the city, it said.
Another four people, including a 14-year-old boy, were shot dead by security forces in neighbouring Hama province on Monday, and two were killed in the northwestern province of Idlib.
Four army deserters died in clashes with loyalists near the Turkish border village of Al-Yunsieh, and explosions were heard amid fighting between deserters and soldiers in the Damascus suburb of Douma.
The Observatory reported similar clashes at Shifunia village near the capital with at least seven people killed, but did not say if they were soldiers or deserters.
On Sunday, the SNC said Homs was besieged and facing an "invasion" from some 4,000 troops deployed near what has become a focal point of the uprising against Assad.
Syrian foreign ministry spokesman Jihad Makdisi said the observer "mission has freedom of movement in line with the protocol" Syria signed with the Arab League.
Under that deal, the observers are banned only from sensitive military sites.
The Observatory charged that the authorities had changed road signs in Idlib province to confuse the observers, and urged them to contact rights activists on the ground.
Opposition groups have said the observers must stop their work if they are blocked by the authorities from travelling to places like Homs.
Syrian Foreign Minister Walid Muallem has said he expects the observers to vindicate his government's contention that "armed terrorists" are behind the violence.
Western governments and rights watchdogs blame Assad's regime for the bloodshed.
Opposition leaders charge that Syria agreed to the mission after weeks of prevarication in a "ploy" to head off a threat by the 22-member League to go to the UN Security Council over the crackdown.
The observers will eventually number between 150 and 200, Arab League officials say.