A powerful earthquake that struck northern Japan, this morning has caused disruption to communications services in the country.

The earthquake struck at 10:13 a.m. just off the coast of Niigata prefecture, which is north west of Tokyo. The magnitude 6.8 quake registered an intensity of 6+ on Japan's scale of 0 to 7, in three locations.

Citing figures collected at 1:30 p.m. Japan's NHK television reported at least two people were killed and more than 190 people injured. Television images showed dozens of collapsed buildings and damage to structures such as bridges. At one point black smoke was seen rising from the Kashiwazaki nuclear plant but the operator said this was from an electrical transformer that had caught fire and the reactors had shut down safely with no release of radiation.

Japan is one of the most seismically active countries in the world and buildings standards are high to avoid widespread damage during strong quakes.

As a result of the temblor major telecommunications carriers have imposed restrictions on phone calls into and out of the affected area.

NTT East, the major fixed-line provider in the area, has activated its “disaster dial 171” service that allows people in the area to leave voicemail messages that can be checked by those in the rest of the country. An internet version of the service, web 171 has also been activated and allows those affected to leave voice, still image and video images for friends.

The major cell phone carriers have similarly restricted calls and activated their own disaster message board services on wireless internet sites.

The restrictions are put on telecom networks to keep lines free for emergency purposes and also to avoid a collapse of the network due to the massive surge in calls often seen after a powerful earthquake.