"Any agreement with North Korea must build on the current test suspension and ultimately include the dismantlement and removal of all nuclear, chemical, and biological weapons from North Korea," the letter said.
The senators also noted that they would seek tougher sanctions on North Korea if Trump's deal wouldn't mandate complete denuclearization and other tough conditions.
US Defense Secretary Jim Mattis said the day before that the sanctions imposed on North Korea could only be lifted after Pyongyang demonstrates that it has taken "irreversible steps to denuclearization."
The statement came a week after Trump abruptly pulled out from the historic planned summit with Kim scheduled to take place on June 12 in Singapore, citing hostile comments made by Pyongyang. At the same time, however, Trump did stress that he was looking forward to meeting with Kim "some day."