Chinese President Xi Jinping left North Korea on Friday after a landmark two-day state visit to the neighboring ally, highlighting their strong alliance amid trade and nuclear tensions with the United States.

Xi's plane left Pyongyang's Sunan International Airport in the afternoon, China's CCTV reported without providing details. He arrived in Pyongyang early Thursday, becoming the first Chinese head of state to visit the North in 14 years.

Xi and North Korean leader Kim Jong-un held summit talks on Thursday at which the Chinese leader said he will help address the North's security concerns, and Kim said he will exercise patience in efforts to resolve the Korean Peninsula issue, according to their state media reports.

It marked their fifth summit since last year and came after they last met in Beijing in January.

Late Thursday, Xi and his wife, Peng Liyuan, along with Kim and his wife, Ri Sol-ju, watched a group gymnastics performance together at Pyongyang's May Day Stadium featuring the strong bilateral ties between China and North Korea, the reports showed.

Details on his schedules on Friday have yet to be confirmed. Xi was expected to pay a visit to the Friendship Tower, a symbolic monument commemorating China's participation in the Korean War in support of the North.

During Thursday's summit, Xi voiced support for the "political" resolution of the Korean Peninsula issue and pledged to play a "positive" and "constructive" role in realizing the denuclearization of the peninsula, the Xinhua news agency reported.

"China, Xi said, is willing to provide assistance within its capacity for the DPRK to address its legitimate security and development concerns ... and play a positive and constructive role in achieving denuclearization of the Korean Peninsula and long-term stability in the region," Xinhua said. DPRK stands for North Korea's official name, the Democratic People's Republic of Korea.

"The international community hopes that talks between the DPRK and the United States will move forward and bear fruit," Xi was also quoted as saying.

Kim said that the North will "stay patient" despite the lack of a positive response to Pyongyang's active efforts to ease tensions and expressed hope that the "relevant party" will work with North Korea to "seek solutions that accommodate each other's legitimate concerns and push for results from the dialogue process," according to Xinhua.

Kim's remarks are seen as an expression of willingness to continue nuclear talks with the US.

Thursday's summit came amid the drawn-out stalemate in denuclearization talks between Pyongyang and Washington following the February summit that ended without a deal.

The February summit collapsed as Pyongyang wanted sanctions relief as a corresponding measure in exchange for dismantling its Yongbyon nuclear complex, while Washington insisted that sanctions should remain in place until the North completely gives up its nuclear weapons program.

The US and North Korea have demanded each side take action first and yield. In April, Kim said that he will wait for the US to make a "courageous" decision until the end of this year.

In apparent frustration over the prolonged impasse, the North fired off short-range missiles in May, raising fears that the move might put nuclear negotiations off track.

Hopes for the resumption of the nuclear talks, however, have risen after the North's leader recently sent Trump a goodwill letter in time for the first anniversary of their first-ever summit in Singapore, and the US president described the letter as "beautiful" and "very warm."

Washington still appears to remain firm that sanctions will remain in place until the North's complete denuclearization and send a message to Beijing that it should play a bigger role in making Pyongyang give up its nuclear weapons.

Just ahead of Xi's trip to Pyongyang, the US announced fresh sanctions on a Russian firm accused of helping the North evade sanctions, in an apparent call for Beijing to cooperate in Washington's pressure campaign to encourage Pyongyang's denuclearization.

The timing of Xi's trip to Pyongyang appears to be well coordinated both for Beijing and Pyongyang, with each of them facing challenges from Washington in trade and nuclear issues.

Experts say that China is trying to its clout over North Korea as a diplomatic card in its intensifying trade disputes with Washington ahead of Xi's meeting with Trump during next week's Group of 20 meeting of global leaders in Japan.

They say that North Korean leader Kim might also use Xi's visit as a chance to draw support and cooperation from his strongest ally before resuming nuclear talks with the US, while also strengthening his internal power base.

On Friday, the North's state media reported on the Kim-Xi summit, saying it will be in their mutual interest to advance their bilateral relations in the face of "serious" and "complicated" international affairs. (Yonhap)