North Korean leader Kim Jong Un has arrived in Beijing at the start of a four-day visit.
Kim left Pyongyang Monday for a visit that is slated to end Thursday, North Korean and Chinese state media reported.
North Korean leader Kim Jong Un and wife Ri Sol Ju inspect an honor guard before leaving Pyongyang for a visit to China on January 7, 2019.
Lu said Kim's visit is a "coincidence" and nothing to do with the China-US trade talks.
The second day of trade negotiations coincided with an unannounced visit by North Korea's Kim for talks with Xi in Beijing, amid speculation of a second meeting between Kim and Trump.
Two of Kim's earlier visits previous year came shortly before historic summits with both South Korea's Moon Jae-In and US President Donald Trump.
"Xi also gains from a summit with Kim - and the timing could not be any better", said Harry Kazianis, director of defence studies at the Center for the National Interest, a United States think tank.
Asked whether China was linking the two issues in an interview Monday with CNBC, U.S. Secretary of State Mike Pompeo said, "The Chinese have been very clear to us that these are separate issues".
Kim may be trying to get a "temperature check" of China's support for sanctions against North Korea, said Harry Kazianis, director of defense studies at the Center for the National Interest, according to the South China Morning Post. But he warned that he may seek an alternative path to a summit with Trump if USA sanctions and pressure against the country continues.
After Kim traveled to China in March 2018 in his first foreign trip as North Korea's leader, Beijing and Pyongyang have deepened their ties.
The Sino-North Korean rapprochement that began a year ago saw both sides emphasizing the enduring historic nature of the relationship between the two countries.
It is hoped that the US and South Korea can accurately understand the constructive significance of amicable China-North Korea ties to push ahead with denuclearization of the Korean Peninsula.
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USA president Donald Trump has threatened a sharp hike in U.S. tariffs on Chinese goods if China didn't yield to his requests. Chinese officials are unhappy with USA curbs on exports of "dual use" technology with possible military applications.
On Sunday, he said negotiations were underway on the location of the next meeting with Kim.
However, outside analysts are highly skeptical that North Korea will easily abandon a nuclear arsenal constructed in the face of deep poverty and likely seen by Kim as his only guarantee of regime survival.
Tuesday also happens to be the reclusive North Korean leader's birthday.
China will "definitely" use North Korea as a bargaining chip to gain concessions from the United States over trade issues, the source said, adding the Trump administration may be forced to review its negotiating policy to "knock down" Beijing.
Last year, he called on the Chinese President ahead of historic meetings with South Korean President Moon Jae-in at the demilitarised zone on their joint border and Mr Trump in Singapore, which resulted in a promise by Mr Kim to work towards denuclearisation.
Washington is pressing North Korea to offer up a detailed accounting of its nuclear arsenal, while the North says it has already done enough and it's time for the U.S.to ease the harsh global sanctions that hold back the North Korean economy.
"China has always supported both sides to continue to hold talks and to reach a positive outcome", Lu said.
When asked whether there can be contacts between the USA and North Korean officials in Beijing, he noted that "these two delegations are not quite of the same level". "The US is very clear about China's stance".
Trump has been asking Xi to prevail on the North Korean leader to give up nuclear programme. "China has actually been a good partner in our efforts to reduce the risk to the world from North Korea's nuclear capability; I expect they will continue to do so".
But relations went through a rough patch in recent years as China backed United Nations sanctions over the North's nuclear tests.
"But Kim Jong Un's visit to Beijing could indirectly show President Trump that although China has maybe less economic cards, China still has some strategic and diplomatic cards", Shi said.