Transport Minister Nariaki Nakayama had come under fire for calling the nation's biggest teachers union a 'cancer' in the education system as well as previously referring to Japan as 'ethnically homogeneous", a comment that angered the Ainu indigenous people who have long suffered discrimination.

Kyodo, quoting sources familiar with the matter, said Mr Nakayama was expected to offer his resignation to Mr Aso on Sunday.

Mr Aso, an outspoken nationalist who favours spending and tax cuts to boost Japan's faltering economy, took office on Wednesday to replace Mr Yasuo Fukuda, the second Japanese prime minister to quit abruptly in a year.

Mr Aso has been expected to call an early election to try to break a policy deadlock due to a divided parliament, where the opposition controls the upper house and can delay laws.

Support for Mr Aso's cabinet fell short of 50 per cent in several media polls published on Friday, lower than Mr Fukuda enjoyed when he took the job, casting doubt whether Mr Aso's can lead his party, which has ruled Japan for most of the past half-century, to an election win.

In a fresh headache for Mr Aso, Mr Nakayama on Saturday called the Japan Teachers Union 'a cancer for Japan's education system' and vowed to destroy the left-leaning group.

'I will stand at the forefront to destroy the Japan Teachers' Union, which is a cancer for Japanese education,' Mr Nakayama told reporters after a meeting of ruling party members in southern Japan.

Mr Nakayama, 65, had already gotten off to a rocky start when he had to apologise and withdraw remarks including a comment that Japan was 'ethnically homogeneous", an assertion that drew protests from Japan's Ainu indigenous people.

The comments sparked calls from the opposition for Mr Nakayama to resign and prompted Chief Cabinet Secretary Takeo Kawamura to warn ministers to watch what they said.

His remarks were also criticised by the ruling Liberal Democratic Party's junior coalition partner.

Asked earlier on Saturday whether he would resign, Mr Nakayama told reporters he would decide after talking with his wife.

Mr Nakayama was a staunch advocate of revising Japan's basic law on education to put more emphasis on teaching patriotism and traditional values. The reforms, opposed by the Japan Teachers' Union, were enacted in 2006.

He also heads a group of LDP lawmakers that declared the 1937 Nanjing Massacre a fabrication. China says Japanese troops killed 300,000 people in what was then the capital, while an Allied tribunal put the death toll at about 142,000. -- REUTERS