U.S. Sen. Cory Gardner has asked the country’s representative to the United Nations to clarify that the Trump Administration’s “maximum pressure” policy of sanctions against North Korea will remain in place until the regime complies with international law.

“We urge you to continue to make clear that United States policy will remain to seek complete, verifiable, and irreversible denuclearization of the DPRK, a longstanding policy goal enshrined in U.S. law, including the North Korea Sanctions and Policy Enhancement Act and the Asia Reassurance Initiative Act,” Gardner wrote to Ambassador Kelly Craft in a Dec. 19 letter.

Five other Republican senators also signed on to the letter. Gardner chairs the Foreign Relations Subcommittee on East Asia, the Pacific and International Cybersecurity Policy.

The Associated Press reported on Monday that a new satellite image dated the same day of the Senate letter shows a new structure at a Pyongyang-area facility at which suspected mobile launchers for ballistic missiles are being made.

The senators were concerned about the possible resumption of North Korea’s long-range missile testing.

On Sunday, the White House’s former national security advisor, John Bolton, said that the “maximum pressure” campaign is largely rhetorical, and that the Trump Administration could do more to deter North Korea if it wanted.