WASHINGTON - The United States has detected a heightened state of alert by Iran's missile forces across the country, a US official told Reuters on Sunday, adding it was unclear whether the higher readiness level was defensive in nature or not.The official, who spoke on condition of anonymity, did not provide further details or say whether the Iranian missiles were taking aim at any specific targets, amid threats by Tehran of retaliation over a US drone strike last week that killed a top Iranian general."They're clearly at a heightened state of alert. Is that heightened state of alert to be better prepared defensively or to be better prepared offensively? That can't be determined at this point," the official said."But we're watching it closely."