The United States will uphold its commitment to Japan’s security while it seeks to implement President Trump’s nuclear deal with North Korea, Defense Secretary Jim Mattis said Friday.

“We’re in the midst of very unprecedented negotiations right now with North Korea, but in this dynamic time, the long-standing alliance between Japan and the United States stands firm,” Mattis told reporters at Japan’s Defense Ministry in Tokyo.

“We are not going to take our alliance with another democratic and free nation into account in this separate negotiation, so it stands firm,” he said.

Mattis spoke at the end of a tour of Asian nations that stand to be affected by Trump’s June 12 deal with North Korean leader Kim Jong Un.

For Japan, the agreement, while welcome in its goal of stripping Pyongyang of its nuclear arsenal, has also generated anxiety about what concessions the United States may make to achieve that objective.

Tokyo’s alliance with Washington, including a U.S. military footprint of about 50,000 troops, is a cornerstone of Japan’s security strategy. But Trump has repeatedly questioned long-standing alliances, including with South Korea and NATO.

The Trump administration, as part of its negotiating strategy with North Korea, has halted military exercises it conducts with South Korea, another key Asian ally, and has suggested that its future troop presence in that nation could be reduced if the threat from North Korea subsides.

The Japanese government meanwhile has expressed concerns about North Korea’s reliability in its commitment to denuclearize, pointing out that it has reneged on previous agreements.

Speaking before a meeting later in the day with Mattis, Foreign Minister Taro Kono said U.N. resolutions against North Korea should remain in place until the country shows progress in ending its nuclear weapons program and takes other steps to reduce the threat it poses to its neighbors.

“There was some step forward in North Korean issues, but I think we really need to keep our relations tight,” he said. “There are a lot of things that still need to be worked on there.”

For Japan, those steps must include North Korea’s giving up not just its nuclear weapons but also its biological and chemical weapons and ballistic missiles of all ranges — not only missiles that could reach the United States.

Mattis did not specifically address chemical and biological weapons in his public remarks but said the United States would maintain its approach of backing up the diplomatic effort with military strength.

“Especially now, we remain vigilant,” he said.

Responding to another key concern of Japan’s, Mattis, after talks with Defense Minister Itsunori Onodera, highlighted the plight of Japanese nationals allegedly abducted by North Korea.

Japan’s government has long sought the return of nationals allegedly taken by the North and held there, sometimes for decades. About 12 such abductees are believed to remain in North Korea, U.S. officials said.