Syrian security forces killed more than 66 people on Sunday, activists said, as the president defended the crackdown on "outlaws" and the pope and Arab League added their voices to condemnation of the violence.
The US envoy to Damascus, Robert Ford, who returned to Syria on Thursday, also said in a US television interview broadcast on Sunday that Washington will "try to ratchet up the pressure" on President Bashar al-Assad's regime.
The latest bloodshed came as Pope Benedict XVI said he was "following with deep concern the dramatic and increasing episodes of violence in Syria that have led to numerous victims and grave suffering."
Ford, who infuriated the Syrian authorities with a visit to the flashpoint protest city of Hama last month, also told ABC news that the violence against protesters was "grotesque" and "abhorrent."
Activists said security forces backed by tanks killed 42 civilians in the eastern city of Deir Ezzor and at least 10 more in the central town of Hula on Sunday.
"Forty-two civilians have been killed and more than 100 wounded in Deir Ezzor by gunfire from the armed forces and security agents," Syrian League for the Defence of Human Rights head Abdel Karim Rihawi told AFP.
Rihawi said that 28 people were killed in Deir Ezzor's Al-Jura neighbourhood and 14 died in Huweika district, adding that "thousands of people have fled the city heading further north."
In Hula in Homs district, at least 10 people were killed in a military assault, Rihawi said. "About 25 tanks and troop carriers entered Hula and carried out military operations," another activist, Rami Abdel Rahman of the Britain-based Syrian Observatory for Human Rights, said earlier.
The Local Co-ordination Committees which organised the protests said on Facebook that snipers were on rooftops in Deir Ezzor "are firing on anything that moves," and also gave a toll of 42 dead.
But Assad roundly defended his security forces.
"To deal with outlaws who cut off roads, seal towns and terrorise residents is a duty of the state which must defend security and protect the lives of civilians," state news agency SANA quoted him as saying.
In contrast to Assad's reference to "outlaws," the pope called on Sunday for an adequate response to the "legitimate aspirations" of the Syrian people.
Ford told ABC he would continue travelling across Syria to observe the crackdown first-hand.
"I don't particularly care (about angering Damascus), because we have to show our solidarity with peaceful protesters," said Ford.
"I'd do it again tomorrow if I have to. I'm going to keep moving around the country. I can't stop."
"We are going to try to ratchet up the pressure," Ford said.
The Arab League also made its first official statement on the unrest, calling on Damascus to "immediately" stop the violence that has rocked the country since mid-March.
Secretary General Nabil al-Arabi also urged an "impartial probe" into the bloodshed, warning against "chaos" and "religious strife" in Syria, it said.
Arabi expressed his "increasing concern" because of "the deterioration of the security situation in Syria following the rise in violence and military operations in Hama, Deir Ezzor and several regions."
On Saturday UN chief Ban Ki-moon phoned Assad to express similar concerns, UN spokesman Martin Nesirky said.
Ban "reflected to the Syrian president the clear message sent by the (UN) Security Council and urged the president to stop the use of military force against civilians immediately."
US and European leaders have pledged to consider new steps to punish Syria after security forces killed more than 30 people on the first Friday of Ramadan, the holy Muslim month of fasting.