Hanoi  - Vietnam is to investigate cracks that appeared on concrete sections built by a Japanese contractor for an underwater highway tunnel in Ho Chi Minh City, a city official said Monday.

Cracks started to appear in early 2008 on all four of the concrete structures, which were slated for the construction of the new Thu Thiem Tunnel under the Saigon River, the Lao Dong newspaper said.

The newspaper said the cracks may ruin the structures, which cost more than two trillion dong (126 million dollars) to build.

"We don't know yet what caused the cracks," said Nguyen Thi Thu Ha, deputy chairwoman of the city's People's Committee.

Huynh Ngoc Si, director of the project management unit, suggested that inexperienced workers, concrete contraction, changes of temperature, and the sinking of the foundations might have contributed to the cracks, Lao Dong reported.

The newspaper reported that the contractor, Japan's Obayashi Corporation, and consultancy firm PCI have suggested pumping chemicals into the cracks to repair them.

The 1.5-kilometre Thu Thiem Tunnel, part of Ho Chi Minh City's East-West Highway project, will have an underwater section of 371 metres, connecting two districts of Vietnam's economic hub.

The two districts are currently linked only by a small, overcrowded bridge.

Several Japanese companies have encountered construction problems in Vietnam recently, including the collapse of the unfinished Can Tho Bridge, touted as Southeast Asia's longest cable bridge, which killed 54 workers and injured 80 others in last September in the Mekong Delta.

The Vietnamese government announced last month that the imbalanced subsidence of the foundation of a temporary buttress was the main cause of the collapse.

So far, no one has been arrested for the collapse of the bridge. A government official said the main contractor, Taisei-Kajima-Nippon Steel of Japan, and subcontractors Nippon Koei and Chodai, had "compensated the victims and their families with the highest responsibility." (dpa)