Hong Kong and Singapore reported more cases on Saturday of a fast-spreading pneumonia that has killed 54 people worldwide as the United States urged Americans to postpone travel to parts of Asia. But Vietnam played down the threat from the deadly virus and said the country was safe for visitors. Severe acute respiratory syndrome or SARS has infected about 1,500 people globally. The World Health Organization believes the disease, which has flu-like symptoms, started in China's Guangdong province last November.