Syrian tanks have rumbled into a rebel bastion in Damascus under the cover of heavy tank and gunfire, activists said, as UN observers toured hotspots in the country. Activists say the Sunday morning raid was launched to crush president Bashar al-Assad's opponents in Douma, an outlying suburb of the captial. A video posted online showed columns of smoke billowing into the sky. Heavy gunfire could be heard in the background. "Regime forces enter Douma each day, but today's assault was the largest," Damascus revolutionary council member Mohammed Saeed told AFP. He said it was "in retaliation for huge anti-regime protests there and because it's a centre of dissent in the Damascus countryside". The Syrian Observatory for Human Rights two civilians and four soldiers were killed in Douma. The violence comes a day after the UN Security Council voted unanimously to dispatch 300 unarmed observers to Syria to monitor a ceasefire that went into effe...