Syrian troops killed 20 people in a tank assault on the eastern city of Deir al-Zor on Sunday
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A resident told Reuters: "Early this morning columns of army tanks and bulldozers, under cover of heavy rounds of gunfire, stormed into the western and northern entrances of the city and dismantled barricades set up by residents.
"A dozen tanks are taking position in the main square in Jubaila market in the northern sector of Deir al-Zor," the resident, who gave his name as Abu Bakr, said by telephone.
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"In a phone conversation with President Assad...the Secretary General expressed his strong concern and that of the international community at the mounting violence and death toll in Syria over the past days," the U.N. press office said.
The military assault on Deir al-Zor, about 400 km (250 miles) north-east of Damascus, was launched a day after U.N. Secretary General Ban Ki-moon told Assad he was alarmed by the escalating violence and demanded he rein in the army.