Nearly 15,000 South Korean and US troops kicked off a 12-day amphibious landing drill Thursday, the largest for two decades, a day after North Korea launched two mid-range ballistic missiles.

Code-named Ssang Yong ("Twin Dragons"), the exercise on the South's southeastern coast will last until April 7 and involve around 10,000 US troops.

"The scale of this year's Ssang Yong is greater than any others in the past," a US military spokesman told AFP.

The US forces will include 7,500 marines belonging to the 3rd Marine Expeditionary Brigade, based in Okinawa, Japan.

Yonhap news agency said 3,500 Marines and 1,000 Navy sailors would take part from South Korea.

It will be the largest joint marine drill of its kind since the annual Team Spirit exercises held between 1976 and 1993.

North Korea views such exercises as provocative rehearsals for invasion and there is a risk they could further fuel already simmering military tensions.

Pyongyang has carried out a series of rocket and short-range missile launches in recent weeks, sparking condemnation from Seoul and Washington.

On Wednesday, it upped the ante by test-firing two mid-range ballistic missiles capable of striking Japan.

United Nations resolutions prohibit North Korea from conducting ballistic missile tests and the UN Security Council was set to hold closed-door consultations Thursday to discuss a possible condemnation of the latest missile launches.

There are nearly 30,000 US troops permanently stationed in South Korea and the two countries stage annual military drills that are routinely condemned by Pyongyang.

Earlier this month, the North's powerful National Defence Commission threatened to "demonstrate" its nuclear deterrent in the face of what it called US hostility.

Seoul's defence ministry warned Wednesday that North Korea could be building up towards a major provocation, but added there were "no signs" of Pyongyang preparing an imminent nuclear test.

North Korea has staged three nuclear tests in 2006, 2009 and 2013.