"Should the sanctions be lifted over a long span of time, this could cause trouble for the North's implementation of the five-year economic development plan," Lee said during a parliamentary forum.

"I reckon that Chairman Kim could agree to a swift denuclearization timeline to attain his economic development goal in 2020 ... before US President Donald Trump's (first) term expires," he added.

At his summit with South Korean President Moon Jae-in, Kim reaffirmed the two Korea's shared goal of "complete denuclearization." But the two leaders' declaration lacked specifics, such as the timeline and sequence of the denuclearization process.

During a surprise summit with Chinese President Xi Jinping in March, Kim said he wants a "phased, synchronous" denuclearization process, which observers took as a sign that he might shun a deal that calls for a quick denuclearization process.

At the forum, the former minister also noted the need for crafting a vision of a nuclear-free zone on the Korean Peninsula based on the premise that Pyongyang will give up its nuclear program and US forces in the South will not introduce strategic nuclear assets in the future.

"A nuclear-free zone on the peninsula will be helpful in improving South Korea's security, and also considerably helpful in reducing tensions when conflicts surrounding the peninsula flare up," he said.

Lee also made the case for multilateral security cooperation in Northeast Asia to move toward "common security," a concept in which states do not bolster their own security at the expense of others'. (Yonhap)