BEIJING -- Chinese President Xi Jinping told North Korean leader Kim Jong Un on Tuesday he hopes Pyongyang and Washington can fully implement the outcome of last week's nuclear summit at which Kim pledged to work toward denuclearization in exchange for U.S. security guarantees. State broadcaster CCTV said Xi told Kim that through the "concerted efforts of the relevant countries" negotiations regarding issues on the Korean Peninsula are back on track and the overall situation is moving in the direction of peace and stability.

The summit between Kim and U.S. President Donald Trump in Singapore marked an "important step toward the political solution of the Korean Peninsula nuclear issue," Xi was quoted as saying in the meeting at the Great Hall of the People in central Beijing.

China hopes North Korea and the U.S. can "implement well the outcomes achieved at the summit," Xi said. China would "as always play a constructive role" in that process, he said.

The U.S. has long looked to China to use its influence with North Korea to bring it to negotiations, but the visit comes as ties between Beijing and Washington are being tested by a major trade dispute.

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Along with a statement signed by Kim and Mr. Trump offering vague commitments to denuclearization and security, President Trump also agreed to suspend military exercises with South Korea in what was seen as a major win for North Korea and its chief allies, China and Russia.

CBS News senior global affairs contributor and Eurasia Group president Ian Bremmer stressed China's role in the region following the summit, calling Beijing "the big winner" in the wake of the historic meeting.

"The most important takeaway here long-term is that the United States is probably going to be a much more marginal player at the end of the day in this region," Bremmer said, pointing to the apparent "freeze for freeze" agreement between Mr. Trump and Kim Jong UN.

"The North Koreans are freezing their ICBM and nuclear tests, and the Americans are freezing our military exercises with the South Koreans. That is exactly the formation that the Chinese have asked for over the course of the past year, and we said absolutely not," Bremmer noted.

Security was tight Tuesday morning at the Pyongyang airport, where another flight was unexpectedly delayed, and later at the Beijing airport, where paramilitary police prevented journalists from shooting photos. A motorcade including sedans, minibuses, motorcycles and a stretch limo with a golden emblem similar to one Kim used previously was seen leaving the airport.

Roads near the Diaoyutai State Guesthouse, where senior Chinese officials meet with visiting leaders, were closed and the same motorcade was later seen heading into the compound. A ring of police vehicles and black sedans surrounded the perimeter of the guesthouse, where Kim stayed on his first visit earlier this year.

A similar convoy of vehicles was seen leaving the state guesthouse in the direction of the Great Hall of the People in central Beijing on Tuesday afternoon. Kim's presence in Beijing and the schedule of his visit, including any meetings with Xi, were confirmed late Tuesday by China's state-run media which broadcast images of the two men together.

Kim was diplomatically isolated for years before making his first foreign trip as leader in March to meet with Xi in Beijing. This is his third visit to China, North Korea's main ally and key source of trade and economic assistance. Following his summit with Mr. Trump, Kim was expected to meet with Chinese leaders to discuss progress in halting his country's missile and nuclear weapons programs in exchange for economic incentives.

China's foreign ministry refused to provide details on Kim's visit other than to say that Beijing hopes it will help deepen relations between the countries.

Geng Shuang, a ministry spokesman, said at a regular briefing Tuesday that the visit would "strengthen our strategic communication on major issues to promote regional peace and stability."

Geng said Beijing supported Russia's calls last week for unilateral sanctions on North Korea -- ones that aren't imposed within the United Nations framework -- to be canceled immediately.

A car believed to be carrying North Korean leader Kim Jong Un drives down Chang'an Avenue towards Tiananmen Square in Beijing, China June 19, 2018. REUTERS

"China always stands against the so-called unilateral sanctions outside the Security Council framework. This position is very clear and we believe sanctions themselves are not the end," Geng said.

While Beijing and Moscow have supported U.N. restrictions, they bristle at Washington imposing unilateral sanctions to put pressure on North Korea.

The Singapore meeting resulted in a surprise announcement of a U.S. suspension of military drills with its South Korean ally, a goal long pursued by China and North Korea. That move is seen as potentially weakening defenses and diplomacy among America's Asian allies, while bolstering China and Russia.

The U.S. has stationed combat troops in South Korea since the Korean War, in which China fought on North Korea's side and which ended in 1953 with an armistice and no peace treaty.

South Korea's Foreign Ministry said Kim's visit to China highlights the "constructive role" Beijing could play in disarming North Korea.

Ministry spokesman Noh Kyu-duk said Seoul and Beijing share a "strategic goal" in achieving the "complete denuclearization" of the Korean Peninsula and that progress in nuclear diplomacy has been facilitating high-level contacts between North Korea and its neighbors.

Noh also downplayed concerns that improving relations between China and North Korea could result in loosened Chinese sanctions against North Korea, saying that Beijing has repeatedly stated its commitment to U.N. Security Council resolutions against the North.

Chinese state media's treatment of Kim's visit departed from past practice of not announcing his travels until Kim returned home. Analysts said Beijing appeared to be trying to normalize such visits.

Cheng Xiaohe, an associate professor at Renmin University's School of International Studies in Beijing, said that unlike previous visits, Chinese state broadcaster CCTV announced Kim's visit before his departure.

"This is an improvement. This shows that China is moving toward a healthier and more normal direction in relations with North Korea," Cheng said. He added that the frequency of Kim's visits was "unprecedented."

Yang Mu-jin, a professor at the University of North Korean Studies in Seoul, said Kim's repeated visits to Beijing this year show that the recent chill in the two countries' ties over Kim's development of nuclear weapons and long-range missiles had fully lifted.

"I believe that indicates that the blood alliance between the North and China has been completely restored," Yang said.

The visit comes as a dispute over the large trade imbalance between China and the U.S. has been escalating, straining ties between the world's two largest economies and moving them closer to a potential trade war.

Mr. Trump recently ordered tariffs on $50 billion in Chinese goods in retaliation for alleged intellectual property theft. The tariffs were quickly matched by China on U.S. exports, a move that drew the president's ire. On Tuesday morning China woke to news that Mr. Trump had directed the U.S. Trade Representative to prepare new tariffs on $200 billion in Chinese products, a move swiftly criticized by Beijing.

A trade war with the U.S. could make it less attractive for China to use its influence over North Korea to help the U.S. achieve its objectives of denuclearization.

"The potential comprehensive trade war will make the cooperation between China and U.S. in North Korea's nuclear issue more complicated," Cheng said. "There will be a big question mark over whether China and the U.S. will continue this cooperation."