SPIN BOLDAK, Afghanistan, Feb 12 (Reuters) - At least 700 Taliban fighters have crossed from Pakistan into Afghanistan to destroy a key dam, a major source of electricity, a provincial governor said on Monday. "We have got confirmed reports that they are Pakistani, Uzbek and Chechen nationals and have sneaked in," Helmand Governor Asadullah Wafa told Reuters by telephone. The dam, Kajaki, has seen major fighting in recent weeks between the Taliban and NATO forces.