North Korea fired two short-range ballistic missiles on Tuesday, Seoul said, Pyongyang's second launch in three days and the first since South Korea and the United States began their largest joint military drills in five years.

Washington and Seoul have ramped up defence cooperation in the face of growing military and nuclear threats from the North, which has conducted a series of banned weapons tests in recent months.

"Our military detected two short-range ballistic missiles fired towards the East Sea from Jangyon area in South Hwanghae province," the Joint Chiefs of Staff said in a statement, referring to the body of water also known as the Sea of Japan. It said the launches took place at 6:41am and 6:51am Hong Kong time.

"Our military has strengthened surveillance and vigilance in preparation for additional launches, while maintaining a full readiness posture through close cooperation between South Korea and the United States."

The launch comes just days after Pyongyang fired two "strategic cruise missiles" from a submarine in an apparent protest over the US-South Korea drills.

Known as Freedom Shield, the drills started on Monday and will run for 10 days.

In a rare move, Seoul's military this month revealed that the two allies' special forces were staging military exercises dubbed "Teak Knife" – which involve simulating precision strikes on key facilities in North Korea – ahead of Freedom Shield.

The Freedom Shield exercises focus on the "changing security environment" due to North Korea's redoubled aggression, the allies have said.

They will "involve wartime procedures to repel potential North Korean attacks and conduct a stabilisation campaign in the North", the South Korean military said previously.

It emphasised that the exercise was a "defensive one based on a combined operational plan".

But North Korea views all such drills as rehearsals for invasion and has repeatedly warned it would take "overwhelming" action in response. (AFP)