Embattled German President Christian Wulff said Wednesday he wanted to stay in office despite a mounting series of scandals weighing on him and his ally, Chancellor Angela Merkel.

"I am pleased to assume my responsibilities (as president), I took them on for a five-year term," Wulff said in a televised interview amid a growing chorus of calls for his resignation.

He added that he hoped by the end of the term in 2015 "to have a record showing I was a good and successful president".

Wulff, 52, landed in hot water last month when the powerful daily Bild reported that he had concealed a home loan at an advantageous interest rate he accepted from the wife of a tycoon friend while premier of Lower Saxony state.

When opposition state deputies asked him whether he had business ties to the tycoon or any firms connected with him, Wulff had kept quiet.

This week it emerged that Wulff had called Bild's editor-in-chief Kai Diekmann one day before the story's publication and left a blistering voicemail message threatening him with "war" if he went ahead with the report.

Meanwhile another publication, Welt am Sonntag, said one of its reporters had been summoned to the presidential palace for a dressing-down over another article about Wulff's strained relationship with his half-sister.

Wulff said in the interview Wednesday that the call to Diekmann had been a "serious mistake" that was "unworthy" of a president and for which he had already apologised.

He said he had felt "helpless" as he was abroad when the report was to be published and had sought an opportunity to clarify the matter before it went to press.

On the loan, Wulff insisted it had been a private arrangement with a friend rather than a business deal.

"I would not like to be president of a country in which you can no longer borrow money from a friend," he said.

Wulff said he would not resign as he had received strong support from Germans in recent weeks calling on him to stay in office.

A spokesman for Merkel, Georg Streiter, said earlier Wednesday she was sure that Wulff would offer a full explanation, as political pressure built on both of them.

"The chancellor is absolutely confident that the president will continue to answer all open questions completely," Streiter told a regular government press briefing. "She has enormous respect for the work of the president."

The media uproar is an unwelcome political distraction for Merkel as she grapples with the eurozone debt crisis at the start of what is expected to be a turbulent year, and a replacement for Wulff would be tough to find.

Sigmar Gabriel, leader of the chief opposition party, the Social Democrats, said it was now up to Merkel to address the scandals buffeting her fellow conservative, whose role is largely ceremonial but who serves as a kind of moral arbiter.

"This is no longer a Causa Wulff, it's a Causa Merkel," he told supporters.

Asked whether he had harmed the office of the presidency, Wulff acknowledged that the recent scrutiny had made it more difficult to serve.

"But I am firmly convinced that I, with a series of things I have done in my term, have strengthened the office of the federal president," he said.

However political scientist Oskar Niedermeyer said that Wulff's statement proved too little, too late.

"You cannot keep making mistakes and then apologising afterwards," he told rolling news channel n-tv.

"The office of the president has indeed been damaged."

Wulff's presidency has been rocky from the start.

His election in June 2010 proved humiliating for Merkel as members of her own coalition broke ranks and refused to vote for him in parliament amid a strong challenge from a former East German dissident, a political outsider.

Wulff, who had once been considered a potential challenger to Merkel, only eked out a victory in the third round.