As stated by Secretary of State, Mike Pompeo, the Trump administration has been enacting strong sanctions against North Korea in an attempt to denuclearize the region and prevent proliferation - known as "the maximum pressure campaign".
The Trump administration's foreign policy with North Korea has been defined primarily by a series of very strong sanctions. These sanctions are meant to pressure North Korea to remove its nuclear weapons programs and prevent proliferation of nuclear weapons to other regions. The US, aligned with Japan, may reconsider this policy after the Singapore Summit if relations between the countries improve. If the negotiations go well, the maximum pressure campaign will no longer be used. If the negotiations do not go well, the campaign will not change. President Trump does not want to use sanctions unless it is necessary.