North Korea fired two short-range ballistic missiles on Monday, South Korea's military said, the latest in a flurry of weapons tests that Pyongyang says are in response to major US-South Korea defence drills.

The two allies were conducting a joint amphibious landing exercise on Monday, days after concluding Freedom Shield, their largest combined military drills in five years.

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

"Our military detected two short-range ballistic missiles fired from around Chunghwa area in North Hwanghae province from 07:47 am (2247 GMT) towards the East Sea," South Korea's Joint Chiefs of Staff (JCS) said, referring to the body of water also known as the Sea of Japan.

The missiles each flew around 370 kilometres (230 miles), it added, calling their launch a "serious act of provocation damaging peace and stability of the international community as well as the Korean peninsula".

The Japanese defence ministry also confirmed the launch, with Japanese media citing officials as saying both missiles fell outside the country's exclusive economic zone.

North Korean state media last week said the US-South Korean military drills called for "stronger war deterrents", including a "multi-faceted and offensive nuclear attack capability".

The North Korean military has carried out multiple drills of its own in recent weeks, including the test-fire of what state media described as an underwater nuclear-capable drone, and its second launch of an intercontinental ballistic missile this year.

State media said the "underwater nuclear attack drone" drill, personally overseen by North Korea's leader Kim Jong Un, was staged "to alert the enemy to an actual nuclear crisis".

Kim also led a two-day drill last week simulating a "nuclear counterattack", according to the official Korean Central News Agency (KCNA).

North Korea's claims have been questioned by experts, who have said they are not the same as credible demonstrations of capability.

But they added that Pyongyang was moving on from simply stockpiling nuclear warheads and attempting to advance and diversify launch mediums.

Analysts had previously said Pyongyang would likely use the US-South Korea drills as an excuse for more missile launches and perhaps even a nuclear test.

'Advanced missile threats'
In the face of growing threats from North Korea, the United States and South Korea have ramped up their security cooperation.

Last week, the US military said it conducted for the first time remote launcher "deployment training" for Terminal High Altitude Area Defense (THAAD) -- a missile defence system -- during Freedom Shield.

"In the face of DPRK's advanced missile threats, the training of our THAAD forces... demonstrates the ironclad commitment to support and defend" South Korea, US Forces Korea said in a statement, using the acronym of North Korea's official name.

North Korea last year declared itself an "irreversible" nuclear power and Kim recently called for an "exponential" increase in weapons production, including tactical nuclear weapons.

Kim also ordered the North Korean military this month to intensify drills to prepare for a "real war".

The South Korean defence ministry said the USS Nimitz, a nuclear-powered aircraft carrier -- was due to arrive in the southern city of Busan on Tuesday.

Both militaries carried out joint naval drills on Monday, which are aimed at strengthening "extended deterrence through the deployment of US strategic assets in the face of growing North Korean nuclear and missile threats", a defence ministry official told reporters.