A Japanese government minister has come under fire from the opposition for saying North Korea should "waste no time" in holding its planned rocket launch so he can campaign in earnest for the upcoming election.

Pyongyang announced last week that it would launch a rocket -- ostensibly aimed at placing a satellite in orbit -- between December 10 and 22, prompting neighbouring countries to guard against a suspected long-range missile flight.

On a visit to his constituency in Osaka on Friday, Chief Cabinet Secretary Osamu Fujimura said government duties were limiting his time to campaign as a candidate for the ruling Democratic Party of Japan (DPJ), ahead of the general election on December 16.

Asked by reporters when he would come home again to put his campaign in full swing, he replied: "Basically, it depends on when the North Korean missile goes up. It will be great if they waste no time and send it up on Monday."

Opposition parties have taken advantage of his apparent gaffe to step up their attack on the centre-left party, widely criticised for being incompetent during its three years in power.

They demanded Fujimura's resignation, saying his duty was to help stop North Korea from launching a rocket.

"We cannot defend Japan with such a chief cabinet secretary. We aim to regain power," former prime minister Shinzo Abe, head of the main opposition Liberal Democratic Party, said on Saturday in a campaign speech in northern Japan.

While Prime Minister Yoshihiko Noda has been touring the country to boost the DPJ's chances in the uphill election battle, Fujimura has stayed in the capital to perform his role as the premier's right-hand man and chief spokesman.

Fujimura was quick to try and control the damage by saying later on Friday: "I want to apologise if a lack of explanation gave a false impression."

He said he had wanted to emphasise his wish to talk with people in his constituency in person and as much as possible.

Japan has ordered its military to shoot down a North Korean rocket if it threatens the nation's territory, readying surface-to-air missiles in Tokyo and Okinawa, and deploying Aegis warships in neighbouring waters.