WASHINGTON — The Obama administration has intensified the American covert war in Yemen, exploiting a growing power vacuum in the country to strike at militant suspects with armed drones and fighter jets, according to American officials. The acceleration of the American campaign in recent weeks comes amid a violent conflict in Yemen that has left the government in Sana, a United States ally, struggling to cling to power. Yemeni troops that had been battling militants linked to Al Qaeda in the south have been pulled back to the capital, and American officials see the strikes as one of the few...