BEIRUT, Lebanon — Syrian insurgents and opposition activists said Monday that rebel forces had taken control of Syria's largest hydroelectric dam, which if true would give them significant control over a vital reservoir and what remains of the sporadic power supplies in the country.The Tabqa Dam, built more than 40 years ago on the Euphrates River in northeast Syria's Raqqa province, provides electricity to areas that are both in rebel and loyalist hands, including the contested city of Aleppo. It would be the third Euphrates dam taken by the rebels, who control two smaller facilities farther upriver.The Tabqa Dam, which the government once boasted had made Syria self-sufficient in power generation, is considered a more potent weapon in the battle for allegiances in the nearly two-year-old Syria conflict. Rebel-held areas have been systematically denied electricity by President Bashar Assad's forces in their effort to turn the population against the insurgency.Claims that the dam was in rebel control came as a possible new confrontation was brewing between Turkey and Syria after a Syrian minivan exploded in Turkish territory at Cilvegozu, an important border crossing. At least 13 people were killed, including three Turkish civilians, and 28 were wounded.The Turkish fatalities were believed to be the first related to the Syrian conflict since October, when a Syrian mortar shell killed five Turks near the border-crossing town of Akcakale, Turkey, eliciting a warning of retaliation by the Turkish government.Turkey's deputy prime minister, Bulent Arinc, did not rule out a bombing or suicide attack as the cause of the Cilvegozu explosion. Syrian rebels, who get military and financial support from Turkey, blamed Assad's government for the explosion.