Could be Chinese.
http://www.dailymail.co.uk/news/article-492804/The-uninvited-guest-Chinese-sub-pops-middle-U-S-Navy-exercise-leaving-military-chiefs-red-faced.html
http://www.popularmechanics.com/military/navy-ships/news/a18094/a-chinese-submarine-stalked-an-american-aircraft-carrier/