Japan is expected to purchase more F-35 joint strike fighters than originally planned in the face of a growing threat from North Korea, according to a senior Republican lawmaker.

"There will be the F-35," Rep. Ted Yoho, R-Fla., a member of the House Foreign Affairs Committee, told the Washington Examiner when asked what kind of equipment the U.S. wants Japan to have.

That prediction lends some detail to President Trump's announcement Tuesday that Japan and South Korea, two critical U.S. allies in the Asia-Pacific region, will be allowed "to buy a substantially increased amount of highly sophisticated military equipment." The sale reflects growing alarm about North Korea's nuclear weapons program and Japan's desire to take more control over its own defense.

Precise details of the promised sale remain unclear. "It's more like a letter of an intent," said Yoho, who chairs Foreign Affairs panel's Asia-Pacific subcommittee. "I think it's an agreement that, ‘yes, We will do this, we will support more arms sales.'"

He couldn't say which side first proposed the military sales, but said the agreements are "mutual" for the two allies. The State Department and the White House declined to comment for this story.

Japan had been set to acquire 42 F-35s, according to Lockheed Martin. Four of those are built at Lockheed's factory in Fort Worth, Texas, and the remaining 38 are to be produced at the Mitsubishi Heavy Industries Final Assembly and Check Out facility in Nagoya, Japan.

Trump's tweet came one day after South Korea said it would allow the United States to deploy the final four launchers to a missile defense battery known as a Terminal High-Altitude Area Defense System, or THAAD.

That decision was a reversal for recently-inaugurated President Moon Jae-in, who halted the deployment of the launchers shortly after taking office. But North Korea's sixth test of a nuclear weapon, which took place over Labor Day weekend, alarmed Moon and also prompted Trump to agree "to lift restrictions on their missile payload capabilities," according to the White House and Moon's office.

The announcements come as Trump's team is mulling a withdrawal of the United States from a free trade agreement with South Korea.

"We have a negotiation we're in," U.S. Trade Representative Robert Lighthizer told reporters in Mexico City on Tuesday. "My hope is that we'll have a successful discussion with the Koreans as things proceed and that the problems with that agreement from our perspective will be worked out."

Such statements have alarmed House and Senate lawmakers, who fear that scrapping the trade agreement would undermine U.S. efforts to boost South Korea's defense in the wake of the rising North Korea threat, by hurting the economy and making South Korea more vulnerable to Chinese economic pressure. Most recently, China has used economic tools in an attempt to induce the new South Korean government to scrap the missile defense system.

"We should be engaging more closely with countries in the Pacific Rim, and developing stronger trade ties with them," Sen. Joni Ernst, R-Iowa, wrote Tuesday in a letter to the president. "If we are not engaging with these nations, there is another economic force in the region that would gladly step in to fill that void and consolidate power, and that nation is China."

Yoho concurred and said a withdrawal from the free trade deal with South Korea could play into North Korean leader Kim Jong Un's hands as well.

"The goal of Kim Jong Un is to drive a wedge between the U.S. and South Korea so that we pull out [of the country], in the hopes that South Korea will collapse like south Vietnam did," Yoho told the Washington Examiner. "That's the goal of Kim Jong Un. And we can't even go down that path."

The timing of Lighthizer's comments, following Trump's tweet, suggests that the military deals and traditional economic sales might operate on separate tracks in the Trump administration.

"The more confusion we give in there and create, the more they're pivoting closer to China, and that's the worst thing we can do to our national security," Yoho said.