Syrian troops and tanks have reportedly advanced deep into the central city of Hama to put an end to weeks of anti-government protests.
Residents said tanks had reached Assi Square in the city centre, focus of mass rallies against President Bashar al-Assad.
There are reports of severe destruction and piles of bodies in the city.
After days of debate, the UN Security Council has agreed on the wording of a statement to condemn the violence.
The 15-member council formally adopted the statement condemning the "the widespread violations of human rights and the use of force against civilians by the Syrian authorities" and calling for those responsible to be held "accountable".
However, Syria's neighbour, Lebanon, while not going as far as blocking adoption of the statement, disassociated itself from the text after it was adopted - a procedure last used decades ago.
The BBC's Barbara Plett at the UN, in New York, says the statement is less than what the European states on the council would have liked, but is more than might have been expected given the strong opposition to saying anything on Syria from some members.
Human rights groups say at least 140 people have been killed in the Syrian unrest since Sunday, mainly in Hama, adding to a civilian death toll believed to be more than 1,600 since March.
Protesters were inspired by the successful uprisings in Tunisia and Egypt. Mr Assad has promised reforms, but blames the violence on "armed criminal gangs" backed by unspecified foreign powers.
Access to events in Syria has been severely restricted for international journalists and it is rarely possible to verify accounts by witnesses and opposition activists.
'Finishing us off'
Activists and residents of Hama, a city of 800,000, said tanks pushed in to the city centre on Wednesday morning, reaching Assi (Orontes) Square.
Earlier, reports spoke of columns of armoured vehicles heading towards the city from where the sound of shelling could be heard.
One resident of the city told the BBC's Damascus correspondent, Lina Sinjab, that he believed a massacre was taking place. He said he had seen piles of bodies in different parts of the city.
There are reports that families trying to flee the city have been shot at to force them to turn back.
Some families who have managed to leave have described the situation as worse than the 1980s, when the late President Hafez Assad, father of the current leader, crushed an uprising, leaving at least 10,000 people dead and the old quarter flattened.
One unnamed Hama resident, who spoke by satellite phone, told Reuters: "The regime is using the media focus on the Hosni Mubarak trial [which opened in Egypt on Wednesday] to finish off Hama."
Communication with the city is all but cut off completely, as are water and electricity, correspondents say.
While about 100 tanks and troop carriers have made for Hama, a further 200 were seen near the eastern town of Deir al-Zour, Rami Abdel Rahman, the head of the London-based Syrian Observatory for Human Rights, told the AFP news agency.
The continuing violence drew a strong response from UN Secretary General Ban Ki-moon on Tuesday.
Mr Assad "must be aware that under international humanitarian law, this is accountable", he said.
"I believe that he lost all sense of humanity," he added.