A Syrian tank and navy assault on the port city of Latakia to crush demonstrations against President Bashar Assad killed at least 21 civilians on Sunday, the Syrian Observatory for Human Rights said.The Observatory said most of the casualties, including tens of wounded, were from big caliber machineguns fired from tanks on the southern quarters of the city, which were also hit by Navy vessels.The Syrian Revolution Coordinating Union said it had the names of 19 people killed.