President Trump tweeted Tuesday morning that he is allowing Japan and South Korea to buy “a substantially increased amount of highly sophisticated military equipment from the United States.”

“I am allowing Japan & South Korea to buy a substantially increased amount of highly sophisticated military equipment from the United States,” Trump wrote on Twitter:

The tweet comes after North Korea conducted its sixth nuclear weapons test over the weekend. Trump did not specify what kind of military equipment the U.S. will sell to Japan and South Korea.

A White House readout of a call between Trump and South Korean President Moon Jae-in on Monday said Trump provided his “conceptual approval for the purchase of many billions of dollars’ worth of military weapons and equipment” from the U.S. by South Korea.

It also said Trump gave his “in-principle” approval to South Korea’s initiative to lift restrictions on their missile payload capabilities. According to South Korean news agency Yonhap, the decision came after weeks of discussions, and is a means to deter North Korean provocations.

South Korea is among the “top customers” for Foreign Military Sales from the U.S. and is an attractive market because of its rising defense spending, according to a recent study by the Congressional Research Service.

Between 2008 and 2016, South Korea spent 75 percent of its total foreign defense purchases on U.S. companies, but also buys from European and Israeli defense companies.

During that time, South Korea FMS contracts with the U.S. totaled $15.7 billion, and commercial buys totaled $6.9 billion, for a total of $22.5 billion.

South Korea is to purchase 40 Lockheed Martin F-35 Joint Strike Fighter from the U.S. for a total of $7.83 billion, with the first delivery scheduled for 2018. South Korea is also to purchase four RQ-4 “Global Hawk” drones at a price of $657 million.

U.S. military sales to Japan dwarfs South Korea by comparison. Japan spends $11 billion per year on Foreign Military Sales, with more than 90 percent of their purchases from U.S. companies, according to another study by CRS.

Recent major purchases include the Lockheed Martin F-35 Joint Strike Fighter, Boeing KC-46 Tankers, Northrup Grumman E2D Hawkeye airborne early warning aircraft, General Dynamics Advanced Amphibious Assault Vehicles, and Boeing/Bell MV-22 Ospreys.