Damage to submarine telecommunications cables from the earthquake in Japan Friday is worse than first believed, with two segments of a trans-Pacific network out of service and at least two other cables damaged.

The western and northern segments of the Pacific Crossing-1 cable, called PC-1 W and PC-1 N, remained out of service on Monday. Pacific Crossing, a subsidiary of Japanese carrier NTT Communications, had a statement on its website that the company was inspecting the damage and accelerating restoration activities.

The southern and eastern segments of PC-1 were still operational. PC-1 is a 21,000-kilometer fibre optic ring that lands at two sites on the US west coast and two sites on Japan's east coast. The more northern landing, at Ajigaura, is between Tokyo and Sendai, which was near the epicentre of the quake.

In addition, PacNet had reported outages on parts of its East Asia Crossing network and Korea Telecom had said that a segment of its Japan-US Cable Network was damaged, according to analyst Stephan Beckert of research firm Telegeography.

On Friday, Taiwan-based Chunghwa Telecom and mainland China carrier China Unicom both reported damage to cables within Asia, but said communication had not been disrupted.

There are 20 cable systems that land on Japan's coast, including both trans-Pacific and intra-Asian networks. All the damaged cables appeared to be ones that come to shore north of Tokyo, at the Ajigaura and Kitaibaraki landing stations, according to Beckert.

Most of the country's cable landing stations are south of Tokyo, and networks located there don't appear to have been affected, he said in an email message. All the networks with landings north of Tokyo also have landings to the south, so no system has had a complete outage, Beckert said.

However, a customer of Hong Kong broadband provider PCCW said trans-Pacific Internet speed had been slashed in the wake of the disaster.

"Access to US sites from Hong Kong incredibly slow, to the point where PCCW has a pre-recorded message in their customer service line apologising for slow Internet service to overseas," the customer wrote. He added that the situation was not as bad as two years ago, after a quake in Taiwan, when overseas Internet access was cut off completely.

An earthquake in Taiwan last year damaged four cables, cutting off service for parts of two days. Internet service in Asia was also disrupted in 2009 when an earthquake in Taiwan and undersea landslides caused by a typhoon cut cables between Taiwan and mainland China.