Japan issued tsunami warnings for parts of the north-east coast on Monday following a powerful aftershock exactly a month after the devastating magnitude-9 earthquake created waves that killed an estimated 28,000 people.

NHK, Japan's public broadcaster, warned of a tsunami up to 2 metres high on the coast of Ibaraki prefecture after the magnitude-7.1 quake.

Although the waves were estimated to be much smaller than those that devastated Japan's north-east coast March 11, the meteorological agency warned people in Ibaraki to evacuate to higher ground.

The warnings were later lifted.

The aftershock briefly forced Tokyo's main international airport to close both of its runways. The epicenter was just inland and about 100 miles north of Tokyo.

It came as the government announced it was widening the evacuation zone around the Fukushima Daiichi nuclear plant due to high levels of accumulated radiation and rising fears about the long-term effects on residents' health.

The operator of the crippled Fukushima Daiichi nuclear complex said the latest quake had no impact on the plant.

The country is still struggling to recover from the damage unleashed just 1 month ago.