(CNN)An old-style green train that is said to resemble one that has transported North Korea's leaders on foreign trips has arrived in China, prompting speculation that Kim Jong Un was in Beijing for talks.
China's mystery visitor: Rumors North Korea's Kim Jong Un is in Beijing
Photos and video of the train at a Beijing train station circulated online late Monday local time. There was also a large security presence at a Beijing guesthouse where North Korean leaders have previously stayed.
The sightings prompted a flurry of speculation that the North Korean leader could be in the Chinese capital ahead of his planned summit with South Korean President Moon Jae-in and a potential meeting with US President Donald Trump.
Kim has not met any foreign heads of state since succeeding his father as North Korean leader in late 2011. Relations with China, a traditional ally, have been strained after Kim purged several top-ranking officials close to Beijing.
According to Japanese broadcaster NTV, the train arrived in Beijing on Monday afternoon. Photos of the carriages were also posted to Chinese social media.
The South Korean government said it was investigating the reports. "Our government is thoroughly monitoring the situation and related movements while closely communicating with the concerned country," a foreign ministry spokesman said.
China's Ministry of Foreign Affairs said it was "not aware of the situation" when asked whether a North Korean official had arrived in the country.
Raj Shah, a spokesman for the White House, said he could not confirm the reports of Kim visiting Beijing when asked about them Monday. "We don't know if they're necessarily true," he said.
He added that the White House was "looking forward to a potential summit some months in advance." The Trump administration previously said they would attempt to arrange the meeting with Kim by the end of May.
Both Kim Jong Il and Kim Il Sung, the current leader's father and grandfather respectively, traveled by heavily armored train when they visited their allies in China and Russia.
The most recent trip was in August 2011, months before Kim Jong Il's death, when he visited Moscow to meet with then President Dmitry Medvedev.
According to Konstantin Pulikovsky, a former Russian diplomat who drew on internal classified memos from Kim's visit to Russia in 2001, the train was staffed by "beautiful lady conductors" and live lobsters were shipped to stops along the route, along with cases of Bordeaux and Burgundy red wines.
During dinners that stretched for four hours or more, guests and conductors sang Russian and Korean propaganda songs, Pulikovsky wrote in his memoirs.
South Korean newspaper Chosun Ilbo reported in 2009 that the train, when used for traveling around North Korea, is flanked by two other trains, one handling reconnaissance and the other security.
"Kim's train is armored and also contains conference rooms, an audience chamber and bedrooms. Satellite phone connections and flat screen TVs have been installed so that the North Korean leader can be briefed and issue orders," the paper said.