Syria's opposition has urged United Nations and Arab League intervention as gunfire from security forces killed 23 people in the besieged central city of Homs.
Syrian National Council (SNC) head Burhan Ghaliun told reporters at a Paris news conference that some of the observers were in the besieged city "but they are saying they cannot go where the authorities do not want them to go."
He sought 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.
The mission is part of an Arab plan endorsed by Syria on 2 November that calls for the withdrawal of the military from towns and residential districts, a halt to violence against civilians and the release of detainees.
Since signing the agreement, Assad's regime has been accused of intensifying its crackdown.
An initial group of 50 observers was to have landed in Syria to oversee the deal aimed at ending a bloody crackdown on anti-regime dissent, which has showed no signs of abating since it erupted in March.
Earlier, the Britain-based Syrian Observatory for Human Rights said "rocket fire and heavy machineguns in the Baba Amro quarter killed 15 people and wounded dozens."
"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.
Seven civilians died in other parts of the central Syrian city and its suburbs, and a woman was killed at Talbisseh near Homs.
Another three people, including a 14-year-old boy, were shot dead by security forces at a demonstration in Khattab in neighbouring Hama province, and a youth was shot dead in Saraqeb in the northwestern province of Idlib.
In other developments, four army deserters died in clashes with loyalist troops near the Turkish border village of Al-Yunsieh, and explosions were heard amid fighting between deserters and soldiers in the Damascus suburb of Douma.
On Sunday, the SNC said Homs was under siege and facing an "invasion" from some 4,000 troops deployed near the city that has become a focal point of the uprising against President Bashar al-Assad.
On Monday, the Observatory said the "observers must head immediately to the martyrs' district of Baba Amro to stop the assassinations and meet with the Syrian people so that they witness the crimes being perpetrated by the Syrian regime."
That demand was echoed by France.
"The Damascus authorities must imperatively, in accordance with the Arab League plan, allow observers access this afternoon to the city of Homs, where the violence is particularly bloody," foreign ministry spokesman Bernard Valero said.
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 last week.
Under that deal, the observers are to be banned only from sensitive military installations.
Ironically, the Observatory said the authorities had changed road signs in Idlib province to confuse the observers, and urged them to contact human rights activists on the ground.
An advance team of Arab monitors arrived on Thursday to pave the way for the observer mission to oversee the deal aimed at ending the crackdown, which the UN estimates has killed more than 5,000 people since March.
Opposition groups have said the observers must stop their work if they are blocked by the authorities from travelling to places like Homs.
"We hold the Arab League and the international community accountable for the massacres and bloodshed committed by the regime in Syria," the SNC said.
General Mohammed Ahmed Mustafa al-Dabi, a veteran Sudanese military intelligence officer who is heading the observer mission, arrived in Damascus on Sunday, a source told AFP.
In a meeting with AFP in Khartoum last week, the 63-year-old Dabi distributed a curriculum vitae detailing a hard core military background, including three years as chief of military operations against the insurgency in what is now South Sudan.
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.
The SNC and rights activists have charged that the government was behind twin suicide bombings in Damascus on Friday that killed 44 people.
Assad's regime blamed the attacks on "terrorist organisations," including Al-Qaeda, although it has not said how it reached that conclusion.
The SNC said "the Syrian regime, alone, bears all the direct responsibility for the two terrorist explosions."
It said the government was trying to create the impression "that it faces danger coming from abroad and not a popular revolution demanding freedom and dignity."
