British Prime Minister Theresa May was attending Japan's top security meeting Thursday, officials said, days after nuclear-armed North Korea fired a missile over the Asian nation.

May, who is in Japan on an official visit, is reportedly just the second foreign leader to attend a meeting of the National Security Council after Australia's then-prime minister Tony Abbott in 2014.

Japan's top government spokesman Yoshihide Suga said her attendance at the meeting underscored the two countries' close ties, after May this week said she was "outraged" by Pyongyang's provocative missile launch.

"It is significant that Prime Minister May was invited to (Japan's) top decision-making meeting for security and diplomacy," Suga told reporters in Tokyo.

The Council, which was created at the end of 2013, consists of Prime Minister Shinzo Abe and key ministers.

Britain wants new UN sanctions against North Korea that would target guest workers sent mostly to Russia and China, and whose wages are a source of revenue for Pyongyang.

The UN Security Council on Tuesday unanimously condemned North Korea over the launch of the missile, which flew over Japanese territory before crashing into the Pacific. 

Early Thursday, May visited a US naval base on the outskirts of Tokyo with Japanese Defence Minister Itsunori Onodera before attending a business forum.

"We have a long history of cooperation in these areas," May said.

"And by the visit today it gives a sign of the growing cooperation and partnership that we have on defence."

Japan's defence ministry on Thursday said it would request its largest-ever annual budget to beef up its missile defence systems.

May arrived in Japan Wednesday with an eye to soothing Brexit fears and pushing ahead on early free-trade talks with the world's number three economy.

She will hold a joint press conference with Abe later Thursday and meet Emperor Akihito on Friday before leaving.

At an informal dinner meeting in Kyoto on Wednesday, Abe and May reconfirmed their cooperation in pressuring North Korea to abandon its nuclear weapons ambitions while asking for China to play a greater role on the issue, according to a foreign ministry statement.

Earlier this year, Britain and Japan signed a defence logistics treaty that allows both country's forces to share equipment, facilities and services.