A powerful earthquake has struck off Japan, shaking buildings in Tokyo for several minutes and forcing people out of their homes, witnesses said.

Japan issued its most serious tsunami warning, saying a wave as high as 6m (20ft) could strike the coast near Miyagi prefecture.

Reports initially reported a quake of magnitude 7.9 but have since been upgraded to an 8.8-magnitude quake, which struck about 250 miles (400km) from Tokyo at a depth of 20 miles.

Tsunami warnings have been issued.