The United States on Wednesday approved a $5 billion sale to South Korea of F-35s, top-of-the-line fighter-jets with stealth capacity, as tensions soar with North Korea.

The State Department informed Congress that it had given the green light to the sale of 25 of the aircraft made by Lockheed Martin as well as engines and related equipment.

The sale "will improve the Republic of Korea's capability to meet current and future threats by providing credible defense capability to deter aggression in the region and ensure interoperability with US forces," a State Department statement said.

"The proposed sale of this equipment and support will not alter the basic military balance in the region," it said.

South Korea has operated F-35s since 2018. The United States only approves sale of the jets to its closest partners, with Turkey booted from the F-35 program after a major purchase from Russia of a missile defense system.

The sale comes as tensions keep spiraling with North Korea, which carried out its latest missile tests just as leader Kim Jong Un visited Russia to discuss greater weapons cooperation with President Vladimir Putin.

The United States has stepped up three-way cooperation with Japan and South Korea, allies that are home to US troops but were frequently at odds over historical disputes until overtures by South Korean President Yoon Suk Yeol.

At a summit last month at the Camp David presidential resort near Washington, President Joe Biden, Yoon and Japanese Prime Minister Fumio Kishida promised to work more closely together on North Korea and other challenges.