If Defence Minister Shigeru Ishiba (above) does step down, it would be the first cabinet resignation since Mr Fukuda took over in September. -- PHOTO: AP

The opposition is demanding the minister's departure after Japan's largest destroyer accidentally rammed a small fishing vessel 10 days ago, leaving two fishermen - a father and his adult son - missing at sea, causing public outrage.

If Defence Minister Shigeru Ishiba does step down, it would be the first cabinet resignation since Mr Fukuda took over in September.

'I'm always ready for that possibility,' Mr Ishiba told reporters when asked whether he would resign.

But Mr Fukuda again stood by the minister, saying in parliament: 'There is no one who knows the current situation at the defence ministry better than Mr Ishiba. I want him to carry out the task of reforming the ministry.'

News reports have quoted anonymous members of the ruling Liberal Democratic Party as saying that Mr Ishiba should voluntarily step down for the sake of the government.

Mr Fukuda has faced a public backlash, with a weekend survey showing approval for his government falling below 30 per cent for the first time since the 71-year-old political veteran took over from the beleaguered Shinzo Abe.

But he has rebuffed opposition demands to call a snap election, at least until Japan hosts the Group of Eight summit of major industrial nations in July. Mr Ishiba appeared to pull back from talk of a resignation when he spoke later in parliament.

'I wouldn't just quit without due consideration,' he said when pressed by the opposition. 'The way to take responsibility, I think, is to answer the feelings of the people who are suffering the most sadness. If I just take it lightly and say that I quit, that means that I'm not addressing their feelings.'

Mr Ishiba, 51, is known for his wonkish knowledge of military affairs and previously served as defence chief when Japan, officially pacifist since World War II, made its landmark deployment of troops to Iraq.

Japan only created a full-fledged defence ministry last year and it has since been beset by scandals. The ministry's former top bureaucrat was arrested last year for allegedly taking tens of thousands of dollars worth of bribes.

The Atago destroyer that crashed into the tuna-fishing boat is equipped with the Aegis radar combat system, seen as a frontline defence against North Korea.

Mr Fukuda on Friday made a fresh apology for the collision, telling parliament: 'I feel that the government bears a keen responsibility. I can't make any excuses for this accident, committed by a naval ship that is supposed to be protecting people's lives and property.'

The opposition has alleged an attempted cover-up because the defence ministry flew the Atago's duty officer to Tokyo for questioning before any official probe was opened.

Mr Ishiba has admitted poor judgement in interviewing the officer but said it was not his decision. The opposition also charged that crew members were drunk aboard the Atago, which was returning from Hawaii.

Mr Ishiba denied the charge, saying: 'The captain of Atago says he never allowed his crew members to drink alcohol except at ceremonial receptions.'

Late on Thursday, Mr Ishiba went on a ship to personally supervise last-ditch operations to try to find the two fishermen. -- AFP