At least seven people, including four children, were killed in a collision between a smuggling boat and a Greek Coast Guard vessel off the coast of Lesvos, the U.N. refugee agency (UNHCR) said Friday. Staff from UNHCR also had to be evacuated from a crowded registration site after violence erupted Thursday as frustrations among refugees and migrants boiled over, according to the agency. Thousands of refugees and migrants continue to arrive in Greece by sea from Turkey, which has increased pressure on the government's already overstretched reception and registration facilities on the island, UNHCR said.