Malaysia said its first infected citizen - a 41-year-old man - had attended a meeting of several international delegations, including those from China where the virus originated, in Singapore in mid-January.
South Korea also reported two confirmed cases of South Koreans who visited the same business conference in Singapore.
A 38-year-old South Korean who came into contact with the Malaysian is infected, Jeong Eun-kyeon, director of Korea Centers for Disease Control and Prevention (KCDC) said on Wednesday.
The two had a buffet meal together with several other Koreans during the conference, Korea’s Joongang Ilbo newspaper reported.
Later on Wednesday, another citizen, a 36-year-old man, who was under self-quarantine since Tuesday, was tested positive for the virus, KCDC said.
Singapore’s health ministry said on Tuesday the Malaysian had attended a business meeting with Chinese nationals at the Grand Hyatt Hotel and it was working to identify others at risk.
Authorities have not commented on the nature of the business gathering nor which industry it involved.
A spokesman for the Grand Hyatt Hotel, Gerald Kheng, confirmed that the Malaysian had stayed at the hotel for a week in mid-January but he did comment on meetings held at the hotel.
The Singaporean health ministry has not told the hotel how, where or when the Malaysian was infected with the virus, Kheng said. The hotel had been deep cleaned and it was not aware of any other cases among its hotel guests or staff, he said.
Singapore - one of the worst hit countries outside China in the 2003 outbreak of Severe Acute Respiratory Syndrome (SARS) - has reported 24 cases of coronavirus, including some local transmission cases.
The city-state says no evidence has emerged of widespread community spread but it announced new precautionary measures on Tuesday including halting group activities such as school assemblies.
Several firms in Singapore have suspended business and media events, including a big travel fair, but the Singapore Airshow is set to go ahead next week albeit on a smaller scale.
The World Health Organisation has said cases of human-to-human transmission outside China are deeply concerning and could signal a much larger outbreak.
The death toll from the epidemic neared 500 on Wednesday.