US PACIFIC COMMAND, Hawaii ― South Korea will buy more U.S. weapons, develop more advanced missiles and receive a more consistent presence of U.S. military nuclear weapons to defend against North Korea, according to agreements reached between top U.S. and South Korean military leaders that concluded in Seoul Saturday.

Both Chairman of the Joint Chiefs Gen. Joseph Dunford and U.S. Defense Secretary Jim Mattis were in Korea during the previous week to discuss ongoing requests by Korea to upgrade its artillery and missile defense capabilities and discuss how to implement President Donald Trump’s agreement with his Korean counterpart to expand the deployment of U.S. bomber, submarine and aircraft carriers to the area to dissuade the North.

The regular meetings between the two countries’ defense heads have occurred since the 1970s, but both Mattis and Dunford conceded during the week that there was little that was regular about the unstable backdrop of North Korea’s rapidly developing nuclear capabilities.

Dunford continued his work on countering North Korea and addressing broader regional threats on Sunday in Hawaii at U.S. Pacific Command. There Dunford hosted Korean Air Force Gen. Jeong Kyeong-doo, the chairman of South Korea’s Joint Chiefs of Staff and Japanese Chairman of the Joint Chiefs of Staff, Adm. Katsutoshi Kawano.

The more rare trilateral approach - rare due to continued political sensitivities between Japan and South Korea - to countering the North has become more feasible due to the aggressive testing by North Korean leader Kim Jong-un, defense officials and analysts said.

“Pyongyang’s behavior explains why [Japan] Prime Minister Abe and [South Korean] President Moon have elevated security cooperation to the top of their agenda, relegating history to the ‘read later’ file,” said Pat Cronin, the Asia program director for the Center for a New American Security.

In Seoul, Mattis said the threat to the U.S. and the region from North Korea’s program had increased significantly even from his previous trip to Seoul earlier this year.

“Our military and diplomatic collaboration has taken on a new urgency,” Mattis said of the pressure the region is under. “In the past few months, the North has conducted two [intercontinental ballistic missile] ... tests, has launched two intermediate-range ballistic missiles over Japan and conducted a sixth nuclear test.”

To that end, the U.S. has opened the door to a host of military reinforcements that seek to deter North Korea. The most visible will be symbolized by a three-carrier sail in the region next month, potentially to coincide with President Donald Trump’s visit there.

Song also said that the meetings in Seoul had focused on the technical details “to expand rotational deployment of U.S. strategic assets.”

The agreement also calls for supporting “the implementation of extended deterrents and commitments” in South Korea, Song said at a joint press conference with Mattis following the meetings.

Dunford said that while the assets assigned to Korea, the greater U.S. Pacific Command area of operations and the number of ships or bombers that could be sent from other theaters are fixed in number, “what’s not fixed is the manner in which we integrate all those three things. So when do we do it? What pattern do we show?”

Dunford said any additional show of force would also be weighed against the potential risk that added presence could encourage Kim Jong-Un to react militarily.

“What kinds of things have proven to cause [Kim Jong-un] to be concerned? What kinds of things do we believe have actually deterred him from doing things in the past?” Dunford said. “What things maybe exacerbate a crisis or perhaps have, you know, been counterproductive?”

The carriers Nimitz, Reagan and Roosevelt entered the Japan-based 7th fleet area of operations last week. It was the second time this year that the U.S. had three carriers in the Pacific at the same time, Dunford said, but the potential to have the three operate closely together would be the first time the U.S. has used its largest symbol of power projection in that way since 2007, the Pentagon said.

Dunford has emphasized that the carriers are not targeting North Korea, but are there to assure regional allies including South Korea and Japan that the U.S. will not pull back on commitments to defend them against a potential attack. Seoul and its surrounding area is well within North Korea’s artillery range and is home to 25 million people, including 28,500 U.S. troops and an estimated 300,000 Americans.

Song also said the visit had further advanced South Korea’s efforts to modernize its military. Neither side offered what those specifics were, but Song said the two sides agreed to “continue expanding the acquisition of high tech capabilities for the [Republic of Korea] military. “

Dunford said that once he returns to Washington he would brief the president to prepare him for his own visit to the region, which begins Nov. 3, and then the Pentagon will work “a whole portfolio of Republic of Korea foreign military sales.”

Notably, Song said the two sides had also made progress on South Korea’s request “to remove the warhead payload limits on the revised missile guidance.”

Since 1979 South Korea has agreed to limits on its payload and range on its missiles in exchange for U.S. assistance on its military technology.

The U.S. would not say what terms were agreed to. When pressed, a U.S. defense official who spoke on the condition of anonymity would only say that “we continue to work with our [Republic of Korea] counterparts on all aspects of the Revised Missile Guidelines. When this work is finalized we will provide additional information.”