Olympics earthquake: Magnitude-6 tremor strikes off the coast of Tokyo causing early morning alarm for Games visitors but no damage or tsunami risk

An earthquake has struck off the coast of Japan causing early morning alarm for Olympians.

The magnitude-6 tremor struck at a depth of six miles in the sea to the east of Tokyo by the Ibaraki Prefecture at about 5.30am local time.

Athletes and journalists in the city for the games reported numerous aftershocks but there was no risk of a tsunami or any damage.

One reporter said the shaking continued for at least 30 seconds and was 'still going' after it struck.

Another said it was 'not a monster' and 'unlikely to be destructive' so he said he was 'going back to sleep'.

Australian journalist Mark Beretta was live on air for Sunrise when the earthquake struck.

He was standing on a 10-storey temporary broadcast tower outside the Olympic stadium.

He said: 'Welcome back to the Olympic city where we are currently in an earthquake, an earth tremor.

'The roof above us is moving and you might notice our lights and camera are moving as well.'

After it stopped, following 30 seconds of shaking, he added: 'That was quite an unusual moment, I have not been through an earthquake before.

'We're in a tower, which at home would be made of scaffolding, but here it is made of construction steel.'

UN NERV, which posts disaster warnings for Japan, tweeted: 'Earthquake Detailed Report – 8/4.

'At around 5:42am, an earthquake with a magnitude of 5.6 occurred offshore Ibaraki Prefecture at a depth of 30km.'

It added: 'The maximum intensity was 2. There is no threat of a tsunami.'

Japan is used to earthquakes, with buildings being designed to withstand the impact of one.

In March the country had a strong one that shook buildings in Tokyo and caused a tsunami advisory for the country's northeast coast.

The magnitude 7.2 quake struck near the east coast of Honshu, the GFZ German Research Centre for Geosciences reported at the time.

On March 11, 2011, a magnitude 9.0 quake sent a tsunami that triggered meltdowns at the Fukushima nuclear power plant.

The disaster heavily damaged coastal neighbourhoods and killed more than 18,000 people, with around 10,000 people killed in Miyagi prefecture.