The trip is believed to be Kim's first known trip outside North Korea since he took power in 2011.
North Korean dictator Kim Jong Un traveled to China for talks with that country's president, Xi Jinping, Chinese and South Korean news agencies said early Wednesday.
There had been speculation that it was Kim aboard a train, believed to be one used by Kim's father, that arrived in Beijing on Tuesday. Xinhua, China's state media agency, did not answer that question, but did report Wednesday local timethat the North Korean leader had met with Xi.
The trip is believed to be Kim's first trip outside North Korea since he took power in 2011.
The South Korean news agency Yonhap also reported that Kim visited China from Sunday to Wednesday along with his wife, Ri Sol Ju, citing North Korea's state-run radio station.
Xinhua called it an unofficial visit. It said Xi held talks with Kim at the Great Hall of the People in Beijing. China is North Korea's neighbor and most important ally.
Xinhua said Kim visited China at the invitation of Xi.
In a statement Tuesday night, White House press secretary Sarah Huckabee Sanders said: "The Chinese government contacted the White House earlier on Tuesday to brief us on Kim Jong Un's visit to Beijing. The briefing included a personal message from President Xi to President Trump, which has been conveyed to President Trump.
"The United States remains in close contact with our allies South Korea and Japan. We see this development as further evidence that our campaign of maximum pressure is creating the appropriate atmosphere for dialogue with North Korea."
The Trump administration has encouraged China to increase pressure on North Korea, and in September China ordered North Korean-owned businesses to close and imposed limits on oil to North Korea.
Kim's visit also comes ahead of an agreementby U.S. President Donald Trump to meet with Kim. When the meeting was announced, a date of sometime before May had been suggested.
Trump and Kim have traded barbs over the last year over North Korean missile tests, including tests of intercontinental ballistic missiles, and its nuclear program. North Korea has conducted six nuclear tests, including one in September.
The United Nations imposed tough new sanctionson North Korea last year. On Tuesday, State Department spokeswoman Heather Nauert said Russia and China can "do more" on North Korea.
Nauert said the department is continuing to proceed with planning, along with other agencies, for a summit between the United States and North Korea.
She said earlier Tuesday that while it had not been confirmed that Kim was on the train that arrived in Beijing, there seemed to be a lot of "fanfare," and that the U.S. would leave it up to China to announce who was visiting.
Referring to the Chinese Communist Party, Xinhua said, "Xi said he appreciated that Kim sent him a congratulatory message after the 19th CPC National Congress on his re-election as general secretary of the CPC Central Committee and the assumption of office of chairman of the CPC Central Military Commission."
China's rubber-stamp Parliament earlier this month passed a constitutional amendmentabolishing term limits, which will enable Xi to serve indefinitely.
Xinhua also said the visit highlights the "great importance" of relations between the two countries. "We speak highly of this visit," Xi told Kim, according to Xinhua.
Using an acronym for the Democratic People's Republic of Korea, North Korea's official name for itself, Xi said, "We are willing to work together with DPRK comrades, remain true to our original aspiration and jointly move forward, to promote long-term healthy and stable development of China-DPRK relations, benefit the two countries and two peoples, and make new contribution to regional peace, stability and development."
Former deputy and acting CIA director John McLaughlin said on MSNBC earlier Tuesday that the reported visit indicates China's nervousness about talks between South and North Korea, and not wanting to be left on the sidelines.
"I've seen the Chinese attitude on this — that is, North Korean behavior and nuclear weapons — evolve from denial in the early part of the last decade, through recognition to concern today," said McLaughlin, who is also an MSNBC national security analyst.
"I don't think they have control over Kim Jong Un," McLaughlin said. "And having just been in South Korea, and having talked to the South Koreans and having seen the gleam in their eye about the idea of meeting face-to-face with the North Korean leader — not that they're naïve about it, but they want to talk."
"So I think anytime the North and South are getting together, it makes Beijing nervous if they're not in the room and they don't have control over it," he said. Beijing definitely wants talks rather than nuclear threats or tests, but "they'd like to have some influence on those talks," he added.
China's state councilor will visit Seoul to brief South Korea's head of national security on Xi's meeting with Kim, South Korea's president's office said.
South Korean lawmaker Shin Dong Uk, who is in a minority party and has been critical of South Korean President Moon Jae-in, said on Twitter that Moon and Trump have been tricked again. Borrowing a popular proverb, he wrote in Korean, "fool me once, shame on you; fool me twice, shame on me."
South Korea's senior presidential secretary for public communications, Yoon Young Chan, said in a statement that China's government informed South Korea's government of the meeting prior to the official announcement.