"We are a long ways away from understanding what happens after Kim Jong-un. That's why I think it's necessary the United States, South Korea, Japan come together in a strong alliance, working with China to develop a plan not only for denuclearization of Kim Jong-un's regime, but what happens next," he said. "It's a very important question that we do not have answered at this point."

Talk of removing Kim Jong-un from power has surfaced in the US as concerns about the North's nuclear and missile threats spiked, with some lawmakers asking questions if it would be a good idea to assassinate the North's leader.

On Sunday, Secretary of State Rex Tillerson was asked in a TV interview if the US administration is considering any assassination plan for Kim, and said he is aware of "no such plans." He also said that the US goal is the North's denuclearization, not regime change.

Gardner called for China to take action to rein in North Korea.

"I hope that in the coming days and weeks we will see emerge from China a plan to deal with Kim Jong-un's nuclear stockpile and how China is going to take responsibility for the situation in North Korea. And I hope we see more along those lines," he said.

US President Donald Trump and Chinese President Xi Jinping held their first summit talks last week and agreed to increase cooperation to persuade the North to abandon its illicit weapons programs, according to the White House.

Tillerson said after the summit that the US is prepared to chart "our own course if this is something China is just unable to coordinate with us," echoing Trump's repeated warning that the US will take on the North on its own unless China helps rein in the North. (Yonhap)