"Our stance is that there should not be dialogue for dialogue's sake," Yun said in his first press briefing since taking office in the new government of President Park Geun-Hye.
"The important thing now is for North Korea to show its sincerity to the international community by implementing its past denuclearisation pledges," he added.
Yun's remarks followed a visit to China by a top North Korean envoy, who brought a personal letter from leader Kim Jong-Un to Chinese President Xi Jinping.
Chinese state media said the letter stated the North's willingness to resume long-stalled six-party talks on denuclearisation involving China, the two Koreas, the United States, Russia and Japan.
The reported message was greeted with scepticism in South Korea, where observers saw it as an effort by the North to appease its key ally Beijing rather than a genuine signal of intent.
North Korea has repeatedly declared that its programme to develop a viable nuclear deterrent is not open to negotiation.
Seoul and Washington, meanwhile, insist that the North must demonstrate its commitment to abandoning its nuclear weapons programme in order for formal talks to begin.
"We need to see North Korea's sincerity in its actions," Yun said, adding that all outward signs were of Pyongyang going in the opposite direction and strengthening its nuclear arsenal.
The North Korean state media's coverage of envoy Choe Ryong-Hae's visit to China made no mention whatsoever of a dialogue proposal.
Of the letter handed to Xi, Pyongyang's official Korean Central News Agency said only that it conveyed Kim's desire to deepen the "traditional friendship" between North Korea and China.
China is North Korea's key economic benefactor and diplomatic protector, but it signed off on UN sanctions punishing Pyongyang for its nuclear test in February.
South Korea's Unification Ministry said North Korea needed to set its priorities straight.
"If the North genuinely wants dialogue, the first step should be responding to our repeated call for working-level governmental talks on the Kaesong industrial complex," said spokesman Kim Hyung-Seok.
Kaesong was the highest-profile victim of two months of elevated military tensions that followed the North's nuclear test.
Established just north of the border in 2004 as a rare symbol of cooperation, the showpiece project had more than 120 South Korean firms employing some 53,000 North Korean workers.
The North barred South Korean access to the zone and pulled out its workers in early April. Seoul withdrew the last of its nationals early this month.
The South has called for talks on securing the raw materials and product stocks left by the departing South Korean managers, but the North has so far refused.
The unification ministry also criticised the North for a recent personal attack on Park Geun-Hye, in which it labelled South Korea's first female president a "confrontation maniac".
"For the sake of healthy inter-Korea relations, the North needs to control and restrain itself... and immediately stop such remarks that are too unspeakable for us to quote here," spokesman Kim told reporters.
Park, who is scheduled to visit Beijing for a summit with President Xi next month, said Monday she would seek to "work more closely with China in order to resolve North Korea issues".