The Department of Homeland Security advised airlines Wednesday of a potential security threat involving explosives in shoes on overseas flights bound for the U.S., sources have told NBC News. Several officials familiar with the advisory said that credible and "very recent intelligence" warns of possible attempts to attack passenger planes that way and that as a result, airlines will be paying extra attention to passengers' shoes on U.S.-bound flights from overseas. According to the officials, the threat information wasn't specific to any particular airline, country or time, and there was no indication of any particular plot, either. "It's a reminder that we are under constant threat and an advisory to airlines be on their A game," one official told NBC News. The officials said the threat seemed unrelated to the Olympic Games in Sochi, Russia.