Syrian rights groups say government troops have killed at least 55 people in assaults on two towns where civilians have protested President Bashar al-Assad's autocratic rule.

The rights groups say at least 42 people were killed Sunday when Syrian forces backed by armored vehicles launched a pre-dawn raid on the eastern town of Deir el-Zour. They say another military assault on the central town of Houleh killed at least 13 people.

President Assad defended his violent crackdown, saying it is a “national duty” to deal with what he calls “outlaws” who cut off roads and “terrorize” people. Speaking during talks with Lebanon's visiting foreign minister Sunday, Mr. Assad also said Syria is on a path to reform.

The Syrian president is facing growing international pressure to end the crackdown. The U.S. envoy to Damascus, Robert Ford, told ABC News that Washington is trying to “ratchet up the pressure” on the president's government. Ford, who is heading back to Syria to observe the crackdown first-hand, called the latest bloodshed “frightful” and “abominable.”

Arab League Secretary General Nabil al-Arabi called on the Syrian leader to stop the violence “immediately” — the first time the pan-Arab body has issued such a statement since the revolt began in March.

The United Nations says Secretary-General Ban Ki-moon gave the same message to Mr. Assad in a phone call Saturday.

Arabi also expressed “growing concern” about the deterioration in Syria's security situation with the escalating violence and military operations in Deir el-Zour and other regions.

Pope Benedict said he is deeply concerned about increasing violence in Syria. In a weekly address to pilgrims at his summer residence south of Rome, Benedict appealed to Syria's government and opposition activists to return to “peaceful coexistence.” He also said the Syrian people's “legitimate aspirations” should be met.

Turkish Prime Minister Recep Tayyip Erdogan said Saturday he has run out of patience with Syria's crackdown and will send Foreign Minister Ahmet Davutoglu to Damascus on Tuesday to deliver that message in a “decisive” manner.

Mr. Assad's adviser Buthaina Shaaban criticized Turkey on Sunday, saying it failed to condemn armed groups who Damascus says have brutally murdered Syrian civilians and security forces. She said Syria will make that point to Davutoglu in an even stronger manner.

The United States says Mr. Assad's security forces are responsible for the deaths of more than 2,000 people in the uprising.