KDDI, Japan's second-largest telecoms operator, said it will send out a ship equipped with remotely-controlled robots as soon as the ground is still.
The robots can dive to a depth of 2,500m to repair the damaged cables - a task that may take months to complete.
In the mean-time, there is enough redundancy in the telecommunications infrastructure linking Japan to the rest of the world to keep the country connected.
KDDI spokesman Shin-ichiro Itoyama, speaking by phone from the company's headquarters in Tokyo, said that the deep water robots had previously only been used to lay cables on the seabed and not for any major repairs as there had never been much damage from previous earthquakes.
More than 5,400 people perished in Friday's 9.0-magnitude quake - the most powerful ever to hit Japan - and the ensuing tsunami.
Some 9,500 people are still missing.
At one point, during the interview, Mr Itoyama broke-off: "Oh wait a second, the ground is shaking again," he said.
Powerful aftershocks continue to rattle the devastated country, reaching magnitudes of 6.0 and higher. Many cables were damaged in the hardest-hit areas, such as Ibaraki Prefecture
The country's biggest operator, NTT, was hit hard as well.
"About half a million of telephone circuits are down," Kazuhiro Gomi, the head of the US branch of NTT, told the BBC.
"Nowadays, the Internet is as important as telephone lines - and about 150,000 internet circuits are down."
Mr Gomi added that mobile phone services have also suffered, especially in the north-east coastal area.
Other companies with undersea cables in the waters around Japan include Australian operator Telstra International, Taiwan's largest phone operator Chunghwa Telecom, and global telecommunications service provider Pacnet, headquartered in Singapore and Hong Kong.
While some of them say their services have been restored, others are still struggling.
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