Defense Secretary Jim Mattis praised the historic peace negotiations between North Korea and South Korea Friday, but stressed that North Korea will still need to earn U.S. and allies’ trust as it works toward denuclearization.

“We will build, through confidence building measures, a degree of trust to go forward. So we’ll see how things go,” Mattis said. “I don’t have a crystal ball. I can tell you we are optimistic right now that there’s opportunity here that we have never enjoyed since 1950.”

Mattis’ comments came less than a day after North Korean leader Kim Jong Un and South Korean president Moon Jae-in signed the Panmunjom Declaration for Peace, Prosperity and Unification on the Korean Peninsula. If its carried through the agreement could lead to denuclearization and the formal end to the Korean War.

Earlier Friday President Donald Trump also hailed the agreement, but said U.S. military, economic and diplomatic pressure would continue as the agreement moves forward.

“We seek a future peace, prosperity, and harmony for the whole Korean Peninsula unlocking, not only a brighter future for the people of Korea, but for the people of the world,” Trump said. “However, in pursuit of that goal, we will not repeat the mistake of past administrations. Maximum pressure will continue until denuclearization occurs.”

Trump is scheduled to meet with the North Korean leader next month.

“I look forward to our meeting,” Trump said. “It should be quite something.”

The peace agreement could have significant ramifications for the estimated 28,500 U.S. troops based in South Korea. When asked, Mattis said it was premature to discuss whether the agreement could lead to a reduction in the number of U.S. forces permanently stationed there.

“That’s part of the issues that we’ll be discussing in negotiations with our allies first, and of course with North Korea,” Mattis said. “So I think for right now, we just have to go along with the process, have the negotiations and not try to make preconditions or presumptions about how it’s going to go. The diplomats are going to have to go to work now.”