In this handout image provided by the South Korean Presidential Blue House, Chung Eui-Yong (R), head of the presidential National Security Office shakes hands with North Korean leader Kim Jong-Un (L) during their meeting on March 5, 2018 in Pyongyang, North Korea.

Events are happening quickly on the Korean Peninsula with planned summits between North and South Korea and the United States, a presidential official in Seoul said on Tuesday, amid reports that North Korean leader Kim Jong Un is in China.

Bloomberg, citing three unidentified sources, reported on Monday that Kim was in Beijing in what would be his first known trip outside North Korea since taking power in 2011. The unconfirmed visit also comes ahead of a potential summit with President Donald Trump.

"The presidential Blue House is watching things in Beijing very closely, while keeping all possibilities open," said the senior official in Seoul, who spoke on condition of anonymity.

Improving bilateral ties between North Korea and China would be a positive sign before the planned summits, he said.

The details of Kim's visit, including its purpose and itinerary, were not yet known, Bloomberg reported. Japanese media reported earlier on Monday that a high-ranking Pyongyang official appeared to have arrived by train in Beijing.

The Blue House official said the South Korean government had been aware of "related movements" in North Korea, such as the train, for a few days but he could not confirm whether Kim or another high-ranking North Korean official was visiting China.

Beijing is secretive and isolated North Korea's main ally, as well as its biggest trading partner.

Kim is due to hold separate summits with South Korea in late April and the United States in May, including a potential meeting with Trump. "The fact that the summits are being held has been beyond our expectations. Right now, the situation surrounding the Korean Peninsula is moving very quickly and it would be inadvisable to think with prejudice," the Blue House official

said.