He said the situation at the quake-hit Fukushima nuclear plant remained grave, a day after an explosion at a reactor.

Japanese Prime Minister Naoto Kan has admitted that his political campaign fund received donations from a foreigner, in breach of Japanese law.

Japanese broadcaster NHK says the total number of confirmed deaths caused by the disaster now stands at 1,596. But police warn that the death toll in Miyagi region alone could top 10,000.

The authorities are stepping up relief efforts as the scale of the tragedy becomes clearer.

Huge numbers of survivors are gathered in emergency shelters, and many people are without fresh running water, heat and power.

Japanese shares plummeted at the open on Monday morning, the first trading day after the massive earthquake that hit Japan's northeast shore.