A powerful aftershock has rocked an area of Japan still reeling from the deadly March 11 earthquake and tsunami disaster, the US Geological Survey said on Friday.

The 6.1-magnitude quake struck at 12:58 am (1458 GMT Thursday), 276 kilometres (171 miles) east of Sendai, Honshu Island, at a depth of 24 kilometres, the USGS said.

There were no reports of any damage or casualties and no threat of a tsunami.

The biggest ever quake recorded in Japan struck on March 11, triggering a huge tsunami and leaving 13,591 people dead, with another 14,497 still unaccounted for.

Tens of thousands of people lost their homes, while many others were forced to evacuate after a series of explosions at the Fukushima Daiichi nuclear plant sent radiation spewing into the air.

The radiation leaks have resulted in bans on produce from the affected area and hurt the fishing and farming industries because of public fears over radioactivity in food.

Workers at Fukushima entered a reactor building Thursday for the first time since an explosion sparked the nuclear crisis.

Wearing gas masks and protective suits with oxygen tanks on their backs, two workers stepped into the building housing reactor number one -- one of four reactors badly damaged at the plant -- to gauge radiation levels.

The company later sent in more workers to set up a ventilation system to filter radioactive material out of the air within the reactor building, the officials said.

earlier related report
Mexico City rocked by 5.8 earthquake
Mexico City (AFP) May 5, 2011 - A magnitude 5.8 earthquake shook Mexico Thursday, causing buildings to sway in the capital some 300 kilometers (187 miles) away from the epicenter, an AFP correspondent and US seismologists said.

The quake struck at 1324 GMT near the town of Ometepec in the state of Guerrero, at a depth of 10 kilometers (six miles), the US Geological Survey (USGS) said.

The tremor rattled Mexico City, where earthquake alarms rang out in some quarters and office workers briefly spilled onto the streets.

Helicopters patrolled the capital -- whose metropolitan area has a population of about 21 million people -- while civil protection authorities reported no immediate damages.

"Mexico City usually feels quakes a bit more because it's built on a former lake bed," USGS geophysicist John Bellini told AFP.

The sediment under the city is largely unconsolidated layers of silt and volcanic clay, and "it behaves like a bowl of jelly," amplifying seismic shockwaves, said Bellini.

Such an effect was at work in Mexico's historic 8.1-magnitude quake of 1985, which occurred off the Pacific coast some 350 kilometers from Mexico City but devastated the capital, killing at least 10,000 people.

Dozens of moderate to strong temblors are recorded each year in Mexico, where movement of the North American plate against the Pacific and other plates makes it one of the most active seismic regions in the world.

A strong 6.5-magnitude quake struck southern and central Mexico on April 7.