The U.S. embassy in Seoul, South Korea tweeted this over night:
B-52 bombers fly over South Korea, once again demonstrating the depth of the alliance twitter.com/usembassyseoul…
— U.S. Embassy Seoul(@usembassyseoul) March 20, 2013
The U.S. embassy's tweet appears to be of a military exercise between America and South Korea.
"United States B-52 bombers carried out simulated nuclear bombing raids on North Korea as part of ongoing U.S.-South Korean military exercises, Pentagon officials said on Monday," Bill Gertz reports.
"Pentagon press secretary George Little told reporters that B-52 bombers from Andersen Air Force Base, Guam, conducted a training mission over South Korea March 8 during war games known as Exercise Foal Eagle."