|Protests have gathered strength in recent weeks despite the crackdown on demonstrators [Reuters]
UN Secretary General Ban Ki-moon has called on Syrian President Bashar al-Assad to halt the use of force against civilians "immediately".
Ban spoke to the Syrian leader by phone on Saturday to express "his strong concern and that of the international community at the mounting violence and death toll" in the country over the past week, the UN press office said in a statement.
The secretary general has been trying for months to speak with Assad, UN officials have said, but the Syrian president had been refusing to take his calls.
The last time Ban spoke with Assad was in May, when he told Assad to end the violent crackdown against pro-democracy demonstrators.
Saturday's phone call comes days after the UN Security Council overcame deep divisions and condemned Damascus' bloody crackdown on civilian protesters, the 15-nation body's first substantive action on Syria's five-month-old uprising.
Ban's statement said Assad referred to "the large number of lives lost among the security forces and police," an assertion that the Syrian government has made repeatedly since the crackdown began - and one which Western diplomats have voiced doubts about.
"The secretary-general said he condemned the violence against both the civilians as well as security forces," the statement said.
Ban also referred to Assad's promises of reforms.
"The secretary-general underscored that for these measures to gain credibility, the use of force and mass arrests must stop immediately," the UN press office said.
On Sunday, activists said the army had launched a pre-dawn assault on the eastern city of Deir ez-Zor.
The London-based Syrian Observatory for Human Rights (SOHR) said tanks entered several areas of the city, and shelling was reported from at least three suburbs.
Meanwhile, the opposition Local Co-ordination Committees (LCC) said defected soldiers in the Joura neighbourhood were trying to protect residents from the military and pro-Assad thugs.
The LCC also reported that the army, backed by tanks, was strengthening its positions in the city of Homs.
The Assad government has sought to crush a democracy movement with brutal force, killing around 1,650 civilians and arresting thousands of dissenters since March, according to the SOHR.
As the violence continued on Saturday, Walid al-Muallem, Syria's foreign minister promised "free and fair" general elections by the end of the year. He said the new parliament will represent the aspirations of the Syrian people.
The four-year term of the current parliament expired earlier this year and Assad is expected to set a date for new legislative elections before the end of 2011.
Al-Muallem pledged to press ahead with reforms and said dialogue was the only way forward for Syrians.
In-depth coverage of escalating violence across Syria
"The ballot box will be the determining factor and it will be up to the elected parliament to review adopted draft bills to decide on them," he said during a meeting he held with Arab and foreign ambassadors in Damascus.
Shortly after Muallem's announcement, security forces arrested Walid al-Bunni, a prominent opposition figure and former political prisoner, the head of the SOHR said.
"The Syrian security services on Saturday evening arrested opposition figure and ex-political prisoner Walid al-Bunni" and his sons Moayed and Ayad, Rami Abdel Rahman told the AFP news agency.
'Patience running out'
Turkish Prime Minister Tayyip Erdogan gave a stark warning to Assad that his country's patience is
running out with Syrian regime over its bloody crackdown on protesters.
"Our patience s running out. Therefore I am sending Turkish foreign minister to Syria to hold all the necessary talks there," Erdogan said.
"Our messages will be conveyed to them clearly during this visit and the upcoming process will be shaped according to their response and steps that will be taken."
He said the minister would go to Syria on Tuesday.
Meanwhile, Guido Westerwelle, the German foreign minister, said in an interview to be published on Sunday, that Assad has no future in politics.
"I don't believe that Assad has a political future ahead of him which is supported by the Syrian people", Westerwelle told the German Sunday newspaper Allgemeine Sonntagszeitung.
Westerwelle's ministry is reportedly in talks with members of the opposition in Syria.
Gulf Arab states on Saturday called for an "immediate halt to violence and bloodshed" in Syria after security forces reportedly killed at least 24 civilians in the latest round of anti-government protests.
The six-member Gulf Co-operation Council (GCC) also expressed concern over the "increasing violence and the excessive use of force which resulted in killing and wounding large numbers."
The GCC called for "resorting to wisdom, and implementing serious and necessary reforms that preserve the rights of the Syrian people, and meet their aspirations."