The opposition Syrian National Council appealed for the Arab League to immediately send observers to the besieged city of Homs and other hotspots of a bloody crackdown on dissent.
The call came a day before a first group of Arab League observers is set to arrive in Syria to begin monitoring a deal the 22-member bloc agreed with the government in Damascus aimed at ending nine months of violence.
"Since early this morning, the Homs neighbourhood of Baba Amr has been under a tight siege and the threat of military invasion by an estimated 4,000 soldiers," the SNC said in a statement received in Nicosia.
"This is in addition to the nonstop bombing of Homs that has been going on for days," said the council, the main umbrella group of opponents of President Bashar al-Assad.
The central city of Homs has been a focal point of the Assad government's crackdown on anti-regime demonstrations, as well as the site of fierce clashes between the army and mutinous soldiers.
A nine-member advance team of Arab monitors arrived in Syria on Thursday to pave the way for the observer mission to oversee the deal to end the crackdown, which the UN estimates has killed more than 5,000 people since March.
"The Syrian National Council demands that the Arab League observers go to Homs immediately, specifically to the besieged neighbourhoods, to fulfill their stated mission," it said in the statement.
"In addition, we demand that the observers go to all the hotspots in Syria, or withdraw and conclude their mission if it is not possible for them to do so."
"We hold the Arab League and the international community accountable for the massacres and bloodshed committed by the regime in Syria," said the opposition group.
Syrian Foreign Minister Walid al-Muallem has said he expects the Arab League observers to vindicate his government's contention that the violence in the country is the work of "armed terrorists."
Western governments and rights watchdogs blame the Assad regime for the bloodshed.
Opposition leaders charge that Syria agreed to the mission after weeks of prevarication in a "ploy" to head off a League threat to go to the UN Security Council over the crackdown.
Muallem met the advance team of Arab League officials on Saturday, in talks the ministry's spokesman called "positive".
Arab League Assistant Secretary General Samir Seif al-Yazal, head of the nine-member advance team, said the first group of observers, more than 50 experts, would leave for Damascus on Monday.
They will eventually number between 150 and 200.
The mission is part of an Arab plan endorsed by Syria on 2 November that also calls for the withdrawal of the military from towns and residential districts, a halt to violence against civilians and the release of detainees.
But since signing the agreement, the Assad regime has been accused of pressing on with its crackdown on dissent.
The opposition SNC and human rights activists have also charged the Syrian government was behind twin suicide bomb attacks on Friday that killed 44 people in Damascus.
Assad's regime has blamed the attacks on "terrorist organisations," including Al-Qaeda, although it has yet to release details on how it reached such a 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."
The Muslim Brotherhood, an influential component of the SNC, issued its own statement pointing the finger at the regime.
"The regime gave a bloody welcome to the team of Arab observers, on the morning of Holy Friday, to cover up the weekly demonstrations across the Syrian map," it said.
"We draw attention… to the fact that Syrian television delayed nearly an hour in broadcasting images of the incident from the time of its announcement, as the theatre director put the finishing touches on the scene, including fake blood."
It noted the speed at which the government blamed Al-Qaeda.
The Syrian Observatory for Human Rights reported at least 21 civilians killed on Friday and 20 people killed by security forces on Saturday.
The bodies of four detained civilians were found on Saturday bearing signs of torture in Hula in Homs province, the Observatory said in a statement received in Nicosia.
The Observatory demanded the Arab League "immediately head to the town of Hula to document this flagrant violation of human rights which is just the tip of the iceberg of what is going on in Syria."