Yun stressed the significance of the two countries’ close bilateral cooperation and China’s greater role in “achieving substantial progress on North Korea’s denuclearization,” his ministry said in a statement.

Kerry in response acknowledged Seoul’s consistent North Korea policy and suggested further cooperation to that end.

In his address to the U.N. General Assembly on the day, Yun urged Pyongyang to forgo its policy of pursuing nuclear weapons and economic development in tandem. He also called for greater international pressure to induce that change, citing Syria’s recent promise to relinquish its chemical arsenal.

"As is the case with Syria, the international community should forge united efforts to roll back DPRK's (North Korea's) nuclear weapons programs to prevent the advent of another nuclear-armed state," the minister said Friday.

"However, if the DPRK decides to give up its so-called parallel pursuit of economic development and nuclear armament, and in turn embarks on a path of genuine change through concrete actions, the Republic of Korea stands ready to help North Korea."

Backed by its prime ally Beijing, Pyongyang has recently called for high-level dialogue with Washington and a restart of six-nation denuclearization talks with South Korea, the U.S., China, Japan and Russia.

Yun’s remarks reflect Seoul and Washington’s firm demand for Pyongyang’s preemptive, stronger commitments than those in the so-called Leap Day deal in 2012 before starting any fresh round of talks.

In February 2012, the North agreed to put a moratorium on its uranium enrichment program, stop atomic and missile tests and let in International Atomic Energy Agency inspectors, in return for 240,000 tons of nutritional aid from the U.S.

During a separate meeting earlier in the day with Chinese Foreign Minister Wang Yi, Yun praised Beijing’s ongoing diplomatic efforts but emphasized that Pyongyang should prove its sincerity with action before the six-party talks could resume.

Wang expressed his expectations of Seoul’s “sustained efforts for inter-Korean reconciliation and cooperation,” Yun’s ministry said.

At a news conference later with Korean journalists, Yun said that though China calls for dialogue to resolve the North Korea issue as the host of the six-party talks, it remains committed to Pyongyang’s denuclearization.

Satellite images and news reports recently indicated that the North likely expanded its nuclear facility in Yongbyon and restarted a graphite-moderated reactor to extract plutonium for atomic weapons.

Despite recent progress, inter-Korean relations also face a rough ride. The North suddenly called off reunions of separated families slated for last week and begun churning out barbs against Seoul.

"It is with utmost regret that the DPRK unilaterally cancelled this reunion, which is a humanitarian event in nature, citing political reasons. Such an inhumane decision cannot be justified for any reason," Yun said in the speech.

"I urge the DPRK to convene the family reunion as soon as possible to ease the pain and suffering of the Korean people as a whole."

By Shin Hyon-hee (heeshin@heraldcorp.com)