The communication glitch came the same day that Prime Minister Yasuo Fukuda installed a new cabinet to revive sagging public support for his government.

"This (Saturday) morning I was watching CNN, and even if I don't understand English that well I saw that something strange was going on," Komura, who retained the post in the new cabinet, told a news conference.

"I therefore contacted (his subordinates) myself," he said, adding that foreign ministry officials "should have shared the information faster" to enable him to make an announcement quickly.

The Japanese foreign ministry said it received the information from the US government on Friday afternoon.

Komura said officials did not notify him because the leak posed no risk for humans or the environment. "But that is not a reason for delay," he said.

Chief Cabinet Secretary Nobutaka Machimura, the government's number two who also retained his post, in return criticised the Foreign Ministry for the delay.

"It is sincerely bad that some media reports preceded (the government's) announcement," he said.

"If the Foreign Ministry received communication from the US government, then it should either report to the Prime Minister's office or make a public announcement," he told reporters.

The Pentagon on Friday announced that trace amounts of radioactivity may have seeped out of US nuclear missile submarine USS Houston during a cruise that included stops in Japan and Guam.

An investigation determined the amount of radioactivity that seeped from a valve was less than half a microcurie, or less than what would be found in a 50-pound (22.6 kilogram) bag of lawn fertilizer.

The seepage was detected on July 17 when water that had collected in a pipe connected to the nuclear engineering plant spilled onto a sailor while the submarine was in drydock in Honolulu, Hawaii, the US Navy said.