Sen. Cory Gardner, R-Colorado, says that North Korea should be required to honor decades-old denuclearization agreements before President Trump meets with leader Kim Jong Un. Gardner told CBS' "Face the Nation" that the administration should insist the country take more concrete steps to fulfill the terms of existing pacts before meeting with the president.

"What we need to see is North Korea actually start living up to some of the agreements, the agreements that they have already made, that they said they would to the United States decades ago," Gardner said on Sunday.

Gardner said the prospect of negotiations between the U.S. and North Korea is in part due to the mounting pressure campaign on the regime's economy. "But what we have to hear more of is how we are going to get to those concrete verifiable steps toward denuclearization before this meeting occurs," said Gardner.

"I've talked a lot about the diplomatic runway -- the length we have left on the diplomatic runway, ways we can work with North Korea, work with China, who's enabled North Korea in many cases, to make sure that we're putting maximum pressure on them," he added. "But we have to have those steps, those real concrete steps, before this meeting occurs, because after this meeting, there's going to be very little left of that diplomatic runway."

South Korea's foreign minister says his president, Moon Jae-in, is confident the talks could lead to the denuclearization of North Korea. Mr. Trump was also confident in the North's commitment to halting its nuclear program ahead of talks, telling supporters at a rally in Pennsylvania on Saturday that "they're not sending missiles up."

Gardner, however, says he would have liked to have seen a broader pause in tests before the U.S. and North Koreans come to the table for negotiations. While South Korea said that North Korea had agreed to impose a moratorium on tests of nuclear weapons and missiles if it holds talks with the U.S., Gardner said that doesn't necessarily mean they'll halt their progress.

"I'd like to see some concrete steps, more than just a cessation of testing, because you can still do computer modeling. Look, the United States is making advances every day on our nuclear program and we're not testing nuclear missiles and nuclear weapons each day, but we're still making advancements," added Gardner.