Syrian National Council (SNC) head Burhan Ghaliun told 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 also 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 it 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 mission is part of an Arab plan endorsed by Syria on November 2 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 on Monday 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.
Heavy gunfire killed 23 people in the besieged city on Monday.
On Sunday, the opposition 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.
The observers are due to oversee a deal aimed at ending the crackdown, which the UN estimates has killed more than 5,000 people since March.
The mission is part of an Arab plan endorsed by Syria on November 2 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.
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