PALMYRA, Syria - Islamic State militants retreating from Palmyra laid thousands of mines that they planned to set off simultaneously as the army moved in, a Syrian officer told Reuters in the ancient city after its recapture from the jihadist fighters.The officer said main streets and side roads in Palmyra had been rigged with explosives weighing up to 50 kg. More than 3,000 had already been safely detonated since government forces, backed by Russian jets, retook the city on Sunday, he said.He did not say why the Islamic State fighters failed to set off the explosives before pulling out, but his assertion echoed comments from Syria's antiquities chief, who said the militants intended to dynamite a greater area of the city's 2,000-year-old ruins than they already had.The officer, who did not give his name, said the bombs left behind were linked so they could go off together."All the government buildings are rigged in a network connected to the Daesh leadership headquarters," he said, using an Arabic acronym for Islamic State. "The idea was that as we enter it would all go off at once, not just bomb by bomb. And there are a really huge number of bombs."