In a conversation about North Korea on Sunday, Sen. John McCain, R-Arizona, said "this is probably one of the most serious" national security crises the United States has ever faced.

He said that three administrations have attempted deals with North Korea in attempts to halt the country's progress toward acquiring -- and delivering -- nuclear weapons, and that he's "all for us going back to the table."

McCain has been one of the more outspoken and vocal critics of the Trump administration's approach to the North Korean regime.

Last week, he issued his own stern warning to the regime, saying that the U.S. needs to send a message to leader Kim Jong Un that if the leader's aggressive actions persist, "the price will be extinction."

McCain said Sunday on CBS News' "Face the Nation" that the need for increased emphasis on missile defense and readiness for both South Korea and the U.S is critical as the U.S. continues to prevent the North from developing a nuclear weapon.

While he said he has "great confidence in our capability to develop counter-missile capability," McCain said, "We don't have that right now."

Senators have failed to reach an agreement during debate on several amendments to the nearly $700 billion National Defense Authorization Act, but are set to vote on the annual defense policy bill on Monday. The bill would include a major hike in military spending, including the proposed budget requested by President Trump for fiscal year 2018.

But McCain said in light of tragic accidents involving the USS Fitzgerald and the USS John McCain, which is named for the senator's father and grandfather, the military is showing signs of being ill-prepared.

"Whenever you cut defense capabilities, the first thing that goes is the training and the readiness, and because that's easy enough to cancel. If it's a new weapons system or something of significant impact, then that's a lot harder," said McCain.

"If we're going to ask young men and women to voluntarily serve in the military, we better -- our part of the bargain is to provide them as much as we can and as much as they need in order to operate in the most safe manner," he added.

McCain also said that in addition to a shrinking military, deaths of service members during training is "not acceptable," but could be fixed in the Defense Authorization Bill.

"The appropriations, the money side, is still on the decline, and we have to fix that," he said.

He added, "When we don't fund them adequately, and I'm sorry I get a little heated about this. When we don't fund them adequately, we put their lives in danger. That's what the service chiefs have told us."

The Arizona senator returned to work earlier this month to begin work on bringing the defense bill to debate on the Senate floor, after spending much of the August recess recovering from surgery following his brain cancer diagnosis. Since McCain was diagnosed with glioblastoma, an aggressive form of brain cancer late last month, he has undergone his first round of radiation and chemotherapy, and spent most of his recess at home with family.