"The crisis sparked by the North's nuclear and missile provocations is reaching its most critical point, and the next three to four months will be the most crucial period," Yoo said during a meeting with senior party members.

"The proposal for a delay at this juncture -- without a pledge (from the North) to cease provocations -- is problematic," he added.

The proposal came as Moon seeks fresh momentum for enhancing cross-border ties strained by Pyongyang's continued provocations, including its Sept. 3 nuclear test and a string of ballistic missile launches, including intercontinental ones.

Yoo also voiced concerns that the US administration's national security strategy, announced Monday, is at odds with Seoul's "three-no" policy stance against the deployment of an additional Terminal High-Altitude Area Defense system, its participation in the US-led missile defense network, and the development of a security alliance with Washington and Tokyo.

"The three-no policy that Seoul has stated vis-a-vis China clearly clashes (with the US national security strategy)," he said. "It was China that has carried out tremendous retaliation against the THAAD installation. ... It is time for Moon to focus on diplomacy with the United States."

Despite Seoul's reluctance to conspicuously align with America's anti-missile program, Washington's latest security strategy stated, "We will cooperate on missile defense with Japan and South Korea to move toward an area defense capability."

The document, which casts China as a "revisionist" power, also said that Beijing's infrastructure investments reinforce its "geopolitical aspirations" though Seoul seeks to join the China-led regional development initiative, called the One Belt, One Road.

Observers said that the document underscores the diplomatic dilemma for Seoul seeking to maintain stable ties with both major powers amid their intensifying rivalry. (Yonhap)