Armoured vehicles are seen stationed in the city of Hama Hama - Arabstoday   Syrian security forces have fired shells in the city of Hama, as prayers ended on the first day of the Muslim holy month of Ramadan. Rights groups say around 130 people have been killed by the authorities since Sunday, most of them in Hama.
The city has been largely controlled by opponents of President Bashar al-Assad since June. Fresh violence is also being reported in the capital Damascus.
The UN Security Council discussed the Syrian crisis late on Monday.
The US and Europe are pushing for strong condemnation of the violence, with US Secretary of State Hillary Clinton saying the international community should "come together behind the people of Syria in this critical time".
The BBC's Jim Muir, in neighbouring Lebanon, says tanks and troops returned to the streets of Hama on Monday, after pulling out following Sunday's violence.
Witnesses said the tanks and troops opened fire in the streets as people taking part in evening prayers on the first night of Ramadan, when Muslims fast in daylight hours.
"The shells are falling once every 10 seconds," one witness told Reuters by telephone.
Another activist in the city told AFP news agency that 10 tanks were "indiscriminately" shelling the suburb of Dawar Bilal.
Six people were also killed in the Erbin district of the capital Damascus late on Monday, as they staged protests after prayers, Reuters news agency reported.
"Security cars and pick-up trucks with machine guns mounted on their beds entered Erbin around midnight and assembled at the main roundabout before branching out in the streets and firing at neighbourhoods," one resident told the news agency.
Another activist said tens of people had been wounded.
Scores of people were killed on Sunday in one of the bloodiest days since the uprising against Mr Assad began in mid-March.
Residents said a hospital was among the buildings coming under attack.
There are reports that some families in the city have buried their dead in parks or in the gardens of their own homes because they are afraid to go out.
International journalists have been denied access to Syria and it is not possible to verify accounts by witnesses and activists.
Government attempts to crush continuing protests across Syria have brought strong international condemnation, with Germany and Italy calling an urgent meeting of the UN Security Council on Monday. Germany is currently a member of the council.
Some council members including Russia and China have so far opposed a draft resolution condemning Damascus, although Moscow has called for an end to "repressions" in Syria.
In a statement, Mrs Clinton called on those still opposing official condemnation to "reconsider their positions".
Turkey's Foreign Minister Ahmet Davutoglu said the timing of the Hama assault during Ramadan sent "a very wrong signal, to the Syrian people to the Muslim world and to the global community".
EU foreign policy chief Catherine Ashton urged the UN Security Council to take a "clear stand on the need to end the violence", and has announced further EU sanctions against members of the Syrian regime.
The Syrian government has promised reforms but says its troops are being attacked by "armed gangs" who are backed by unspecified foreign powers.
On Monday, Mr Assad praised the military for "foiling the enemies" of the state, the official news agency reported.