By David Brunnstrom

WASHINGTON (Reuters) - British fighter planes visiting Japan will fly over the South China Sea and Britain will sail aircraft carriers in the Pacific once they are operational in 2020, given concerns about freedom of navigation there, Britain's ambassador to the United States said on Thursday.

The envoy, Kim Darroch, told a Washington think tank that British Typhoon aircraft currently deployed on a visit to Japan would fly across disputed parts of the South China Sea to assert international overflight rights, but gave no time frame.

Speaking at an event also attended by Japan's ambassador to Washington, Darroch said that most future British defense capacity would have to be directed toward the Middle East, but added:

"Certainly, as we bring our two new aircraft carriers onstream in 2020, and as we renew and update our defense forces, they will be seen in the Pacific.

"And we absolutely share the objective of this U.S. administration, and the next one, to protect freedom of navigation and to keep sea routes and air routes open."

In spite of Britain's preoccupations in the Middle East, "we will try to play our part" in the Pacific, he said.

Four British fighter planes arrived in Japan in October to take part in exercises with Japanese forces at a time of rising tensions over China's pursuit of disputed territory in East Asia, including the South and East China Seas.

In Beijing, Chinese Foreign Ministry spokesman Geng Shuang said all countries had freedom of navigation and overflight in the South China Sea, in accordance with international law, and there was no dispute about that.

"We hope the relevant party can earnestly respect regional countries' efforts to safeguard peace and stability in the South China Sea," he told a daily news briefing.

A commentary on the official Chinese news agency Xinhua took a stronger tone, saying UK-China ties could be hurt.

"Such remarks create the impression that London may soon deviate from a largely aloof attitude over the South China Sea issue and start playing a meddling role there like the United States and Japan," it said.

"Should a British warplane embark on a so-called "freedom of navigation" mission in the South China Sea, it would only serve to further complicate the issue and weigh on thriving China-Britain ties."

Japan's ambassador, Kenichiro Sasae, said the United States, Japan and Britain discussed greater naval cooperation at a meeting at the Pentagon in October and Tokyo welcomed greater British involvement in Asian security.

Darroch said British Prime Minister Theresa May and U.S. President-elect Donald Trump discussed the importance of all NATO members meeting their defense spending commitments in a telephone call this week, their second since Trump's Nov. 8 election.

Darroch said all NATO states had committed to spending at least 2 percent of GDP on defense, yet only five, including the United States and Britain, were doing so.

"I think the criticism ... during this election campaign that a number of NATO countries aren't doing everything they can ... is entirely fair and we will see how the incoming administration wants to take that forward," he said.

Trump has criticized European NATO members for not meeting their spending commitments and has also called on U.S. Asian allies Japan and South Korea to pay more for their defense or risk the alliances.

Trump has said he plans to build up the U.S. military, and advisers have said he will pursue a policy of "peace through strength" in the Pacific in the face of China's growing assertiveness.

The advisers say Trump can also be expected to take a more "robust" approach to naval operations to assert navigation rights in the South China Sea, a vital global trade route.

(Additional reporting by Michael Martina in Beijing; Editing by Jonathan Oatis and Clarence Fernandez)