Syrian forces have killed nearly 142 people, including at least 100 when the army stormed the flashpoint protest city of Hama to crush dissent on the eve of Ramadan, political activists say.
A witness in Deir ez-Zor told Al Jazeera that government forces launched fresh attacks on the town early on Monday morning.
"Military forces stormed the city from the west side and 25 people are killed and more than 65 injured," the witness said.
"Artillery and anti-aircraft weapons are being used. The situation in the city is very bad, and medical and food supplies are low."
Deir ez-Zor, Syria's main gas and oil-production hub in the east, has become a rallying point for protests along with Hama.
Sunday's attack on Hama was one of the "deadliest days" since the protests erupted, Rami Abdel Rahman, the head of the Britain-based Syrian Observatory for Human Rights, said.
Death tolls provided by the observatory and other human rights groups showed at least 142 people were killed across Syria, most of them falling in Hama.
"The number of those wounded is huge and hospitals cannot cope, particularly because we lack the adequate equipment," Abdel Rahman quoted a Hama hospital source as saying.
He said the crackdown on Hama came after more than 500,000 people rallied in the city on Friday following Muslim prayers during which a cleric told the congregation "the regime must go".
Western powers condemned the violence amid warnings from Berlin and Paris of fresh sanctions against the government of President Bashar al-Assad.
UN Secretary-General Ban Ki-moon called for an end to the violence and reminded Syrian authorities that "they are accountable under international human rights law for all acts of violence perpetrated by them against the civilian population".
Germany's UN Mission said late Sunday it has asked for a Security Council meeting on Syria and expects consultations to be held on Monday afternoon.
The Syrian authorities have consistently accused "armed gangs" and fundamentalist Salafist Muslims of stirring the unrest and aiming to sow chaos in the Sunni-majority country.
Residents in Hama said the army entered the city with tanks early on Sunday before gunfire erupted, in an apparent operation to wrest back control after security forces withdrew almost two months ago.
Syria has banned most foreign media and restricted coverage, making it difficult to confirm events on the ground.
But interviews with witnesses, protesters and activists painted a grim picture Sunday of indiscriminate shelling and sniper fire as residents fought back by erecting barricades and throwing firebombs at their assailants.
The official SANA news agency said that gunmen shot dead two security forces in Hama while a colonel and two soldiers were "martyred" in Deir ez-Zor.
SANA said the gunmen torched police stations and attacked private and public property in Hama, adding that soldiers tore down barricades and checkpoints set up by the armed men at the city's entrance.
In 1982, an estimated 20,000 people were killed in Hama when the army put down an Islamist revolt against the rule of Assad's late father, Hafez.
Earlier this month, the president replaced the governor of Hama after a record 500,000 protesters rallied in the opposition bastion on July 1 calling for the fall of the regime.
The United States and France enraged the government earlier this month when their ambassadors travelled to Hama in a trip designed to demonstrate solidarity with demonstrators.
At least 1,583 civilians and 369 members of the army and security forces have been killed since mid-March in Syria, according to the observatory.