The opposition Syrian National Council appealed on Sunday for the Arab League to send observers immediately to the besieged city of Homs and other hot spots 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 more than 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," said the SNC, the main umbrella group of opponents of President Bashar al-Assad.
"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 added in a statement.
The Syrian Observatory for Human Rights reported that security forces pounded Baba Amro with mortar and heavy machinegun fire, killing an undetermined number of people and wounding 124 others.
It earlier reported one civilian killed in the city's Karm al-Zeitun district.
The Observatory said 26 people were arrested and tortured in public in nearby Rastan, and that one civilian was killed by security forces in eastern Deir Ezzor province.
It also reported five civilians shot and wounded in the capital's southern district of Sitt Zeinab.
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 deserters.
"Three children who had been wounded on Saturday in military and security operations in the town of Al-Quriya died," it added, noting that reports of the deaths were delayed as the town is cut off by security forces.
A nine-member 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.
"We demand that the observers go to all the hot spots 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," the SNC said.
Foreign Minister Walid Muallem has said he expects the Arab League observers to vindicate his government's contention that the violence is the work of "armed terrorists."
Western governments and rights watchdogs blame Assad's regime for the bloodshed.
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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."
League Assistant Secretary General Samir Seif al-Yazal, heading 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 November 2 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 SNC and human rights activists have charged that 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 not said 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 Observatory reported at least 23 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, it said.
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."
The plight of Syrians was a focus of Pope Benedict XVI's Christmas Day prayers.
"May the Lord come to the aid of our world torn by so many conflicts... May he bring an end to the violence in Syria, where so much blood has already been shed," he told pilgrims in Vatican City.