"The Department of State requests all non-essential official US government personnel defer travel to Japan and also urges US citizens to avoid tourism and non ssential travel to Japan at this time," said a travel advisory issued by the department.

The department said that many roads had been damaged in the Tokyo area and in northern Japan, and strong aftershocks were likely to continue for weeks.

The advisory contained no mention of a feared meltdown of two reactors at a quake-hit nuclear plant in northeastern Japan where more than 10,000 were feared dead.

An explosion at the ageing Fukushima No. 1 atomic plant blew apart the building housing one of its reactors Saturday, a day after the biggest quake ever recorded in Japan unleashed a monster 10 etre (33-foot) tsunami.

The atomic emergency widened Sunday as the cooling systems vital for preventing overheating failed at a second reactor, and the government warned there was a risk it too could be hit with a blast.