DAMASCUS — Syrian forces backed by tanks killed at least 27 people on Sunday, activists said, as President Bashar al-Assad defended what he termed the state's duty to crack down on "outlaws" despite mounting condemnation.

Security forces killed at least 20 civilians in the city of Deir Ezzor, activists said, and at least seven more died in Hula, a town in Homs province, in a separate assault with tanks.

"The operations are focused on Al-Jura district where the army and security forces opened fire, killing at least 20 and wounding dozens of others," Syrian League for the Defence of Human Rights head Abdel Karim Rihawi told AFP.

While the pope joined an international chorus of concern over the bloodshed, Assad defended his security forces' deadly crackdown on anti-regime protests as the "duty of the state" to confront "outlaws."

"Syria is on the path to reforms," he said, quoted by state news agency SANA.

"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," he said.

His statement came a day after a personal appeal by UN chief Ban Ki-moon for Assad to halt the bloodshed.

Ban in a telephone call "expressed his strong concern and that of the international community at the mounting violence and death toll in Syria over the past days," UN spokesman Martin Nesirky said late Saturday.

The secretary general "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," Nesirky said.

Ban's call followed a pledge by the US, French and German leaders 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.

In contrast to Assad's reference to outlaws, Pope Benedict XVI called Sunday for an adequate response to the "legitimate aspirations" of the Syrian people.

"I am following with deep concern the dramatic and increasing episodes of violence in Syria that have led to numerous victims and grave suffering," the pontiff said in a weekly address to pilgrims outside Rome.

Hundreds of tanks and armoured cars have been deployed in Deir Ezzor, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, adding many residents had fled in fear of the assault on the largest city in east Syria.

Syria's government has sought to crush the democracy movement with brutal force, leaving at least 2,059 people dead, including 391 members of the security forces, according to the Britain-based Syrian Observatory.

On Saturday, security forces in Damascus arrested prominent opposition figure and former political prisoner Walid al-Bunni and his two sons, said Abdel Rahman.

Foreign Minister Walid Muallem said earlier the same day that "free and transparent" elections to a new parliament would be held by the end of 2011, as he met with ambassadors posted to Damascus.

Muallem stressed "the commitment of the Syrian leadership to the continued reform process and implementation of measures announced by President Assad."

The embattled president issued a decree on Thursday allowing opposition political parties.

In April, he ordered an end to five decades of draconian emergency rule and abolished the feared state security courts, while in June he said talks could lead to a new constitution and end his Baath party's monopoly on power.

But the concessions have been largely dismissed by the opposition as a ploy to stave off world condemnation.

The Syrian protesters have been calling for greater freedoms since mid-March, inspired by Arab uprisings that ousted the autocratic leaders of Tunisia and Egypt at the start of the year.

Assad's regime has become increasingly isolated, with allies like Turkey and Russia calling for an end to the bloody crackdown.

Turkey's Foreign Minister Ahmet Davutoglu is to visit Syria on Tuesday with the message that Ankara "has run out of patience" with the ongoing violence, Prime Minister Recep Tayyip Erdogan said.

Assad's adviser Boussaina Shaaban said Davutoglu would himself "receive a firm message" in Damascus.

Turkey had failed to condemn "the brutal murders carried out by armed terrorist groups against civilians and soldiers" in Syria, she said, adding that Damascus had always "opposed interference in internal affairs" of other states.

The foreign ministry, meanwhile, rejected criticism from the Arab monarchies of the Gulf and called for them to instead condemn "the violence of armed groups... to allow time for the reforms" proposed by the authorities.