"The government is aware of signs related to North Korea's possible second nuclear test. We cannot exclude the possibility of a second test," the government official told Reuters on Tuesday.
Taro Aso, Japan's foreign minister, said on Tuesday that he had heard reports of preparations for a second test.
He told a news conference: "We have heard [such reports], but I cannot talk about details."
On Monday, US television networks NBC and ABC had said that US spy satellites had detected suspicious activity near the site of North Korea's nuclear test that may signal preparations for another test.
Condoleezza Rice, the US secretary of state, on the eve of a trip to the region to try to stiffen the resolve behind UN sanctions on North Korea, said she hoped Pyongyang would not conduct a second nuclear test.
Rice dismissed scepticism among some in Washington about China's commitment to tough action against its communist neighbour following the sanctions resolution adopted unanimously by the UN security council on Saturday.
She said: "I am not concerned that the Chinese are going to turn their backs on their obligations.
"I don't think they would have voted for a resolution that they did not intend to carry through on."
Nicholas Burns, the US under-secretary of state, said China was already taking action to check goods crossing into North Korea. "The Chinese now are beginning to stop trucks at the 1,400km border and inspect all of them," he said on CNN.
China has made it clear that it is worried that tough action could provoke a collapse of the impoverished and highly militarised state, and its UN ambassador again clarified the limits of its action on Monday.
"Inspections yes, but inspections are different from interception and interdiction," Wang Guangya, the Chinese ambassador, told reporters, in an apparent reference to stopping cargo at sea, which is one of the UN sanctions agreed on Saturday.