The rowers disappeared after the competing in the race on the Tonle Sap river just outside Phnom Penh on Friday evening. Rescuers were not confident they would find the five recent university graduates alive.
They "may be gone already," said Nhim Vanda, vice chairman of the National Committee for Disaster Management, who was leading the 150-man search party.
"We will keep on looking for them until we find them," he said.
There were 22 people on the boat when it capsized in the swirling current, and police boats rescued 17 of them immediately, Nhim Vanda said. A police officer said late Friday there had been 23 people on board.
Singapore Embassy officials could not be reached for comment.
Information Minister Khieu Kanharith said the Singaporeans had just completed the dragon boat race and were being towed to a pier when the accident happened.
"At the pier, they asked the tow boat to untie the rope to let them go on rowing for leisure. They hit a swirling spot, and the boat capsized," said Chea Sokhom, one of the organizers of the festival.
The Singaporeans was among eight teams from Southeast Asia that took part in the event, he said.
Some 440 colorful dragon boats, mainly from across Cambodia, were taking part in the event to celebrate Cambodia's annual water festival, marking the start of the rice harvesting season and the time of the year when the Tonle Sap reverses its flow from north to south.