North Korea has started submarine drills and stepped up preparations for nationwide military exercises, which may be timed to coincide with annual joint drills by South Korea and the United States set to start next week, military sources said Wednesday.

The latest move comes as Pyongyang has ratcheted up its hostile rhetoric against Seoul and Washington as the two allies last week launched a two-month field training exercise called Foal Eagle.

Separately, joint forces will conduct computer-simulated drills named Key Resolve from March 11 to 21. 

On Tuesday, Pyongyang threatened to scrap the 1953 armistice that ended the Korean War, citing tensions over South Korean-U.S. joint military exercises and the United Nations' move to impose sanctions for its Feb. 12 nuclear test.

 A statement by the North's military said the armistice will be "completely" nullified from March 11, when Key Resolve gets into full swing in the South, warning a "precise" strike at any time.

"North Korea's submarines have entered military maneuvers in the Yellow Sea and East Sea," a military source said, on the condition of anonymity. "This year's winter drill was more intense than in the past years and artillery exercises tripled."

When President Park Geun-hye was sworn into office on Feb. 25, North Korea's artillery unit carried out firearm training targeting the South Korean capital Seoul, he said. 

The North will start large-scale drills across the nation from next week when South Korean and the U.S. troops start the two-week war game, another military source said, asking for anonymity.

A large-scale firearm drill will be held near the east coastal city of Wonsan, while nationwide military maneuvers will be carried out, the source said.

According to multiple military sources, the North has recently declared a "no-fly zone" in the west and the east of the Korean Peninsula during the May 11-20 Key Resolve training period, fueling speculation that it may fire off missiles. Seoul's defense ministry declined to comment. (Yonhap News)