The opposition Syrian National Council has appealed  for the Arab League to immediately send observers to the besieged city of Homs and other areas where the Syrian government has used military force to stamp out dissent.

"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.

"This is in addition to the nonstop bombing of Homs that has been going on for days," the council, the main umbrella group of opponents of President Bashar al-Assad, said.

The central city of Homs has been a focal point of the Assad government's crackdown on nine months of anti-government demonstrations, as well as the site of fierce clashes between the army and former soldiers.

Spotlight In-depth coverage of escalating violence across Syria

Al Jazeera’s Zeina Khodr reported from Antakya, a Turkish city near the Syrian border, that intense shelling of Homs had continued throughout Saturday night.

"This is a fresh assault, before the arrival of the Arab League monitors," Khodr said, explaining that Syrian opposition activists believed the government was attempting to crush the rebellion before the delegation visited.

"Activists do not believe monitors will be able to see the situation on the ground," she said.

An advance team of Arab League monitors arrived in Damascus on Thursday to pave the way for an observer mission to oversee a 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 fulfil their stated mission," the council said in its statement.

It continued: "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."

Walid Muallem, the Syrian foreign minister, 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 groups 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.