Beijing's embassy in Tokyo warned that anyone obstructing China's reunification would pay a "heavy price" as the US and Japan hold biennial joint drills reportedly featuring a mock conflict over Taiwan.

American warships take part in a three-day maritime exercise between the US and Japan in the Philippine Sea on January 31. Tokyo’s growing security alliance with Washington has strained its ties with Beijing in recent years.

Citing anonymous government sources, Japanese media reported that Washington and Tokyo had named Beijing as a "hypothetical enemy" for the first time during this year's "Keen Edge" exercise. According to the reports, this year's drill centres around a Taiwan contingency.

"If anyone insists on meddling in China's internal affairs and obstructing China's reunification, then it will not just be about the issue of a so-called hypothetical enemy - they will have to pay a heavy price," the embassy said in a written statement on Tuesday.

According to the US Indo-Pacific command, the Australian Defence Force also joined the eight-day exercise, which will wrap up on Thursday.

Reports also said the drills used real, unaltered maps for the exercise, breaking a norm of using maps that differ slightly from real ones.

Beijing said it expressed its "grave concerns" to Tokyo following the media reports, while Japan denied the accuracy of the drill details described in the reports, according to the Chinese embassy's statement.

Beijing sees Taiwan as part of its territory, to be reunited by force if necessary.

Like most countries, Japan and the US - two of the island's close allies - do not recognise Taiwan as an independent state. But they are opposed to any change in the cross-strait status quo, and Washington is committed to supplying Taipei with weapons.

Washington and Tokyo are reported to have completed a draft plan late last year for a joint operation for a possible Taiwan conflict.

"No one should underestimate the solid determination, firm will and strong capacity of the Chinese people to defend their national sovereignty and territorial integrity," the embassy said, noting that some forces in Japan had exaggerated the situation in the Taiwan Strait and provoked confrontation.

In recent years, Japanese politicians have repeatedly stated that "a Taiwan contingency is a Japan contingency" - a phrase that has irked Beijing. The slogan, first uttered by former Japanese prime minister Shinzo Abe, indicates Tokyo is likely to get involved if Beijing attacks Taiwan, but Japan has avoided explicitly saying it would help defend Taiwan.

Ties between Beijing and Tokyo have been strained in recent years over long-standing territorial issues, the Fukushima water release, Japan's growing concerns about security in the Taiwan Strait as well as its security alliance with Washington aimed at keeping Beijing in check.

A trilateral leaders' summit between China, Japan and South Korea is expected to resume early this year, but there have been no announcements yet on when exactly it will take place. It is expected that Chinese Premier Li Qiang and Japanese Prime Minister Fumio Kishida will hold talks on the sidelines of the event.

The Keen Edge joint drill between Washington and Tokyo is a command post exercise held every other year, alternating with the biennial "Keen Sword" field training exercise.

The US Indo-Pacific Command said increased coordination with the US Space Command and US Cyber Command would also be a feature of this year's Keen Edge exercise.