South Korea's military said Thursday it would hold its largest-ever joint air drill with the United States as tensions mount over a series of threats from North Korea.

The twice-yearly Max Thunder exercise -- to be held from Friday to April 25 -- will be the largest-ever involving 103 aircraft and 1,400 troops, the air force said.

Seoul's F-15K jet fighters will take part along with US Air Force F-15 and F-16s and US Marines' FA-18 and EA-18 aircraft, it said in a statement.

"The combined air forces will strengthen their battle readiness under the current situation when tension rises over the Korean peninsula," it said.

The exercise will focus on "practical scenarios" involving precision attacks on enemies or supply drop missions for troops infiltrating enemy territory, it said.

Separately, the allies are also holding annual Key Resolve and Foal Eagle exercises which last from late February to April 18.

North Korea has slammed the drills as a rehearsal for invasion. In a pointed protest, Pyongyang has launched a series of rockets and missiles in recent weeks, capped by its first mid-range missile test since 2009 on March 26.

The two Koreas also traded fire across the tense Yellow Sea border on March 31, after the North dropped some 100 shells across the border during a live-fire drill, prompting the South to fire back.

The rare exchange of fire came a day after the North warned that it might carry out a "new" form of nuclear test -- a possible reference to a uranium-based device or a miniaturised warhead small enough to fit on a ballistic missile.

South Korean President Park Geun-Hye this week called for tighter vigilance against the North, days after its leader Kim Jong-Un warned of a "very grave situation" on the peninsula.