BEIJING/SEOUL -- Chinese President Xi Jinping's special envoy to North Korea returned home Monday from a four-day trip to the rogue state without meeting its leader, Kim Jong Un, a signal that Pyongyang's nuclear and missile development could lead to a further cooling in relations.

The envoy, Song Tao, met with two of Kim's close aides, Choe Ryong Hae and Ri Su Yong, after arriving Friday in North Korea. China's state-run Xinhua News Agency reported that Song spoke with leaders of the North's ruling Workers' Party of Korea but gave no indication that he met with Kim.

"I have no more details to offer about the specifics of [Song's] visit," Chinese Foreign Ministry spokesman Lu Kang told reporters Monday.

Though Pyongyang's official Korean Central News Agency reported on Song's return, it did not say whether the envoy spoke with Kim.

But a source familiar with the matter told The Nikkei that the two "did not meet," citing Song's relatively low rank within the Chinese Communist Party and Beijing's support for United Nations sanctions on the North.

China sent a representative from its 25-member Politburo to North Korea following the Communist Party congress in both 2007 and 2012, and those envoys met with the top North Korean leader. But Song serves as a member of the party's lower-ranking Central Committee.

Though the Xinhua report said both sides expressed a desire to bolster ties, Kim's snubbing of the Chinese envoy could cause tension between the neighboring countries.

"The two sides also exchanged views on relations between the two parties and the two countries, and on the Korean Peninsula issue and other issues of common concern," Xinhua said. Song is believed to have called for the "double-freeze" approach, under which Pyongyang suspends arms development while the U.S. halts military drills in the area.

But North Korea has shown little interest in abandoning its nuclear program. Han Tae Song, the North's ambassador to the U.N. in Geneva, told Reuters in an interview that Washington has "never accepted" halting its military drills. "If they accept such things, then we will think [about] what we do in the future," he said.

China's party-affiliated Global Times curbed expectations for Song's trip in a recent editorial, saying the U.S. "overestimated Beijing's leverage on the North" and that "Song is not a magician."