New satellite images may have revealed secret missile bases in North Korea.
The Center for Strategic and International Studies say maintenance and minor improvements have been seen at the sites.
The United States has been trying to convince North Korea to give up its nuclear weapons and long-range ballistic missiles.
“The pressure campaign will continue. The sanctions will remain in full force until we achieve the final, fully verified denuclearization of North Korea. The United States, Japan and the world will accept nothing less,” said Vice President Mike Pence.
None of the apparent missile sites have been acknowledged by North Korea.
The country’s leader, Kim Jong Un, pledged to work toward denuclearization during a summit with President Trump in Singapore.