The Syrian army has launched a pre-dawn assault on Deir al-Zour, the largest city in the east and scene of frequent protests, human rights activists say.
Rami Abdel Rahman from the Syrian Observatory for Human Rights says scores of tanks and armoured vehicles have entered several areas of the city reports the BBC.
Other activists said people were unable to flee because troops were all around, but also that some of the soldiers had defected rather than shoot at people.
Earlier, the UN Secretary General told Syrian President Bashar al-Assad to stop using troops against civilian protesters.
Activists say at least 1650 civilians have been killed since uprisings began in March.