China said Thursday it will take firm actions if Japan increases its military prowess in the South China Sea, most of which is claimed by Beijing. The comments came just a day after Reuters reported Japan's plan to send its biggest warship to the disputed waterway to train with the United States Navy in May.

The Izumo helicopter carrier, commissioned only two years ago, will reportedly be sent to Singapore, Indonesia, the Philippines and Sri Lanka before joining the Malabar joint naval exercise with Indian and U.S. naval vessels in the Indian Ocean in July. The three-month-long training is being touted as the “biggest show” of Japan’s navy since World War II.

"If Japan persists in taking wrong actions, and even considers military interventions that threaten China's sovereignty and security. ... then China will inevitably take firm responsive measures," Foreign Ministry spokeswoman Hua Chunying said at a regular press briefing, according to Reuters. However, Hua did not mention if China had received confirmation of the plan.

Read: Are China And Japan At War Over The South China Sea?

"The aim is to test the capability of the Izumo by sending it out on an extended mission," a source told Reuters about Japan's plans Monday. "It will train with the U.S. Navy in the South China Sea."

The U.S., Japan, and South Korea have conducted military exercises and drills in the region due to China’s militarization in the disputed islands and also over North Korea’s latest missile tests.

Brunei, Malaysia, the Philippines and Taiwan also have conflicting claims to the waters, through which over $5 trillion of maritime trade passes annually. While Japan does not have any claim to the waters, it has a separate maritime dispute with China in the East China Sea.

Tensions with China have escalated as Japan invited Philippines President Rodrigo Duterte to visit the warship when it stops at Subic Bay just west of Manila, another source told Reuters.