DUBAI, United Arab Emirates — Border guards in Saudi Arabia have fired machine guns and launched mortars at Ethiopians trying to cross into the kingdom from Yemen, likely killing hundreds of the unarmed migrants in recent years, Human Rights Watch said in a report released Monday. The rights group cited eyewitness reports of attacks by troops and images that showed dead bodies and burial sites on migrant routes, saying the death toll could even be “possibly thousands.” [time-brightcove not-tgx=”true”] The United Nations has already questioned Saudi Arabia about its troops opening fire on the migrants in an escalating pattern of attacks along its southern border with war-torn Yemen. A Saudi government official, speaking on condition of anonymity as he was not authorized to speak publicly, called the Human Rights Watch report “unfounded and not based on reliable sources,” without offering evidence to support the assertion.