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 increasingly provocative banned weapons tests in recent months.

"North Korea fires unidentified ballistic missile towards the East Sea," the Joint Chiefs of Staff said in a statement, referring to the body of water also known as the Sea of Japan.

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 Monday and will run for 10 days as part of the allies' drive to counter North Korea's growing threats.

In a rare move, Seoul's military this month revealed that the allies' special forces units 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 said.

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

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

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

- More to come -

Last year, North Korea declared itself an "irreversible" nuclear power and fired a record-breaking number of missiles.

Leader Kim Jong Un earlier this month ordered his military to intensify drills to prepare for a "real war".

Washington has repeatedly restated its "ironclad" commitment to defending South Korea, including using the "full range of its military capabilities, including nuclear".

South Korea, for its part, is eager to reassure its increasingly nervous public about the US commitment to so-called extended deterrence, in which US military assets, including nuclear weapons, serve to prevent attacks on allies.

Analysts previously said North Korea would likely use the drills as an excuse to carry out more missile launches and perhaps even a nuclear test.

"More missile launches with variations in style and scope should be expected, with even a nuclear test. More acts of intimidation from North Korea should not come as a surprise," said Chun In-bum, a retired South Korean army general.

It is also an opportunity for Pyongyang to try and show that its "reason for developing missiles is for self-defence purposes," said Go Myong-hyun, a researcher at the Asan Institute for Policy Studies in Seoul.