The bridge was being built across the heavily-travelled Hau River, a branch of the Mekong River.
A local police officer said that at the time of the accident there were some 100 workers under the bridge and at least 200 on it. Although Japanese companies are involved with the project as contractors, most of the workers are locals. Some 10 Filipinos and Japanese are reported missing.
The rescue service said flaws in the bridge's design might have caused the accident. Vietnamese TV speculated that heavy rain may have weakened the foundations.
The Japanese Bank for International Cooperation in Hanoi said a 25 billion yen (US$218 million) loan had been provided by Japan to cover 85 percent of the project cost, with the rest to be provided by the Vietnamese government.
Construction on the 2.75-km (1.7-mile), four-lane suspension bridge started in 2004.
