AMMAN (Reuters) - Syrian tanks surrounded a town near the border with Iraq's Sunni heartland on Sunday after tens of thousands, emboldened by defections among security forces, took to the streets there denouncing President Bashar al-Assad, residents said. Assad, from the minority Alawite sect, has sent troops in to towns across the country to try to end four months of protests against his rule. But activists say discontent is growing within the mostly Sunni army rank and file. Killings, mostly carried out by ultra loyalist units, are leading to limited defections within the military, which is controlled by mostly Alawite officers who ultimately answer to Assad's feared brother Maher, activists say. Syria's fractured opposition is also taking steps to unite, forming a 25-member National Salvation Council composed of Islamists, liberals and independents at a meeting in Istanbul on Saturday and agreeing to work towards a democratic vision. More than 1,400 civilians have been killed since the protests began in March, human rights organisations say. Some 1,000 troops and security forces backed by tanks and helicopters surrounded Albu Kamal overnight, a poor eastern border crossing with Iraq, a day after Military Intelligence agents there killed five protesters, including a 14 year old boy, residents said. The killings drove thousands into the streets, overwhelming soldiers and secret police.