The United States, South Korea, and Japan have reportedly started a three-day joint military exercise on the Korean Peninsula to simulate countering a North Korean submarine-launched missile attack.

The exercise began off South Korea’s southern coast near Japan on Monday, the South’s Defense Ministry said. Several naval destroyers and helicopters used in anti-submarine warfare along with more than 800 troops are participating in the drill.

Seoul said the exercise was aimed at “ensuring effective response to the North’s submarine threats including the submarine-launched ballistic missiles (SLBM).”

The drill began a day after US President Donald Trump warned that Washington would act unilaterally to deal with Pyongyang’s nuclear and missile programs if China, the North’s main ally, proved unwilling to help.

A two-seater F/A-18F Super Hornet is seen landing on the deck of the Nimitz-class aircraft carrier USS Carl Vinson during a South Korea-US joint military exercise in seas east of the Korean Peninsula on March 14, 2017. (Photo via AFP)

Trump told The Financial Times that Beijing “has great influence over North Korea…Well, if China is not going to solve North Korea, we will. That is all I am telling you.”

Pyongyang has so far conducted five confirmed nuclear tests and numerous missile launches.

China, which is also North Korea’s key economic benefactor, often backs North Korea but has had its own reservations about Pyongyang’s missile and nuclear programs. Beijing has already taken steps to increase pressure on Pyongyang over the matter.

The North faces international pressure, including an array of sanctions, to abandon its arms development and nuclear program. Yet, it says the programs are meant to protect the country from US hostility.

The US, which has military forces in South Korea, has recently started the installation of an advanced missile system at an air base in the South, further stoking tensions with the North, as well as China.