It is the third time this year that the cable has been damaged.
The Asia-America Gateway (AAG) submarine cable was cut at 11:26pm, according to the AAG management centre.
The damaged cable was found off the coast of Hong Kong (China).
TheVietnam Post and Telecommunications Group (VNPT) said that thebreakdown had reduced internet connectivity of telecom providers betweenVietnam, Hong Kong and the US by about 40 percent.
Internet providers are trying to use more broadband to minimise the effects.
The details were confirmed by Nguyen Van Khoa, Director of FPT Telecom.
When another breakdown occurred in July, most internet providers use similar techniques to deal with the breakdown.
FPTTelecom is working with AAG units to repair the damage. However, itwarned that as Hong Kong was being hit by tropical storm Kalmaegi, therepairs team would need more time to repair the damage, said Khoa.
In July, the cable was found cut 18km off the coast of Vung Tau city in the south at a depth of 19m.
The 20,000km cab le, which connects Southeast Asia to the US was also damaged and later repaired in February.-VNA