Singaporean police on patrol to ensure the security outside the St. Regis hotel where DPRK leader Kim Jong-un has stayed on Sunday. [Photo/VCG]

Journalists prepare outside the St. Regis hotel where DPRK leader Kim Jong-un has stayed in Singapore on Sunday. [Photo/VCG]

A TV news reports about U.S. President Donald Trump is projected on tv sets at a media center for the summit between the U.S and North Korea in Singapore, June 10, 2018. [Photo/VCG]

U.S. President Donald Trump (1st L) arrives in Singapore on June 10, 2018. (Xinhua/Ministry of Communication and Information of Singapore)

U.S. President Donald Trump (2nd R) arrives in Singapore on June 10, 2018. (Xinhua/Ministry of Communication and Information of Singapore)