Syrian forces killed at least six civilians early on Tuesday in an assault on the eastern Damascus suburb of Erbin, residents told Reuters news agency by phone.
"Tens of people are wounded. Six are dead, of whom three have been identified," an activist in the suburb said.
The latest report comes after activists said at least 150 people were killed across the country in the last two days.
Reacting to the violence, European powers on Monday relaunched a dormant draft UN resolution to condemn Syria, circulating a revised text at the Security Council.
Following the hour-long closed-door meeting, several diplomats said that after months of deadlock over Syria in the council, the fresh violence appeared to be pushing the divided members towards some form of reaction.
But envoys disagreed over whether the 15-nation body should adopt the Western-backed draft resolution or negotiate a less binding statement.
On Monday, six people were reported killed in Hama amid shelling by army tanks, with two more killed in al-Bukumal and Deir ez-Zor in the east.
Residents of Hama say the military resumed indiscriminate shelling and firing as residents were breaking their daily dawn-to-dusk fast on the first day of the Muslim fasting month of Ramadan.
The attacks appeared aimed at preventing mosque gatherings during special evening prayers, which
security forces feared could trigger large anti-government protests.
A witness in Deir ez-Zor told Al Jazeera that government forces launched fresh attacks on the town early in the 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.
Activist group Avaaz cited unconfirmed reports on Monday that the resort town of Zabadani, near the Lebanese border, was also under seige by the army.
They said the military had surrounded the area and was preventing people from entering or exiting the area with checkpoints throughout the town.
Hillary Clinton, US Secretary of State, on Monday urged the UN Security Council to condemn Syria over its deadly crackdown on pro-democracy protesters.
"We call on those members of the United Nations Security Council who have opposed any Security Council action that would call on Assad to stop the killing to reconsider their positions," Clinton said in a statement.
A resident from Deir ez-Zor said fresh attacks were launched on Monday morning
Russia, Syria's key European ally, urged Damascus on Monday to immediately stop the use of force and repression against civilian protesters, in its strongest criticism yet of the government's crackdown on demonstrations.
The foreign ministry in a statement expressed "serious concern" over Sunday's violence.
"The use of force against both peaceful civilians and representatives of state structures is unacceptable and should be stopped immediately," the foreign ministry said.
"We are calling on the Syrian government and the opposition to demonstrate the utmost restraint, renounce provocations and repressions and observe the rule of law and respect for international human law."
Sergei Strokan, political editor at the online newspaper Kommersant, told Al Jazeera the statement was "the most strong-worded criticism [of Syria] ever voiced by Russian officials, but it is carefully worded".
"Russia found itself in quite a difficult position after this crackdown," Strokan said.
Abdullah Gul, the Turkish President, expressed "shock" over Syrian security forces' use of heavy weapons, including tanks, to quell civilian protests in the central Syrian city of Hama.
"The recent developments in Syria have deepened our already existing concerns," Gul said. "The footage from yesterday's events has horrified us."
The European Union also condemned the latest violence and expanded its sanctions against Syria on Monday, imposing asset freezes and travel bans against five more military and government officials.
The EU decision brings the number of individuals targeted by the EU to 35, including President Bashar al-Assad.
Four government entities are also on the list. The EU said it would release the names of the new additions to the list on Tuesday.
EU foreign policy chief Catherine Ashton said, "I wish to remind the Syrian authorities of their responsibility to
protect the population.
"The brutal violence creates a serious risk of escalating tension and factional divisions and is not consistent with broad reforms.''
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.
In-depth coverage of escalating violence across Syria
Death tolls provided by the observatory and other human rights groups showed at least 142 people were killed across Syria on Sunday, with at least 100 of them 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 after Muslim prayers during which a religious leader told the congregation "the regime must go".
The Syrian authorities have consistently accused "armed gangs" and fundamentalist Salafi Muslims of stirring the unrest and aiming to sow chaos in the Sunni-majority country.
Syria has banned most foreign media and restricted coverage, making it difficult to confirm events on the ground.