EU commission chief Jean-Claude Juncker has warned Greece against electing "extreme forces" into power and said he would prefer "known faces" - so far the strongest intervention of the EU top brass in the Greek campaign.

"I think that the Greeks - who have a very difficult life - know very well what a wrong election result would mean for Greece and the eurozone," Juncker said during an Austrian public tv debate with EUobserver and several other Brussels-based journalists.

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He steered clear of explicit political advice ahead of presidential elections in Greece next week but said: "I wouldn't like extreme forces to come to power."

The presidential elections - to be held in the Greek parliament on 17 December - could trigger early parliamentary elections, if there are three failed attempts to elect a president.

The far-left Syriza, which wants Greece's debt erased and an end to austerity measures, is topping the polls. Markets are already jittery at the prospect of Syriza leader Alexis Tsipras coming to power.

Asked if Syriza and Tsipras qualify as "extreme forces", Juncker replied: "I would like Greece to be ruled by people who have an eye and a heart for the many little people in Greece and who also understand the necessity of European processes."

"Each party who stands for election has to live up to these standards and I won't comment on the chances of one or the other party, but I would prefer if known faces show up," he added, in an apparent reference to Greece's former EU commissioner, Stavros Dimas, who is standing for election on behalf of the ruling coalition.

In a showcase example of being the more "political" entity that Juncker promised, the EU commission has all but endorsed Dimas.

“The decision can help remove uncertainties around markets, it is a strong signal to Europe that prime minister Samaras put forward his candidate Stavros Dimas, a former commissioner and a convinced European,” said EU commission spokesperson Annika Breidthardt on Wednesday.

Breidthardt denied taking sides in the election, but said that "at this point we feel like we want to make a statement about the Greek forthcoming elections".

France and LuxLeaks

Juncker also admitted that the decision to give Italy and France three extra months to fix their national budgets was "political" and a proof that his commission is not a "machinery run by bureaucrats who are blind to national matters."

"We would have been massively criticised if we had applied the pact simply like that. We would have a totally different discussion now," he said.

He conceded that his reputation has been dented by the LuxLeaks scandal (revealing tax avoidance schemes for multinationals in Luxembourg while he was PM), but insisted that he did nothing illegal at the time.

"I had contact with those firms [Skype, Amazon] when we tried to diversify the structure of Luxembourg's economy. But I never got involved in the tax rulings, because that is not allowed for a Luxembourg finance minister, the law prohibits that he uses his influence on specific tax files," he said.

Asked what he talked to Amazon about, if not about a favourable tax regime, he said: "We spoke about infrastructure, pipelines, what kind of qualified staff we can provide, how we can train them. But we didn't talk about the precise tax questions, because it is not something for the government, it is for the tax administration."

"I am responsible for everything that happened in Luxembourg during my time as prime minister, I've said it and I repeat it. I am constantly forced to defend myself as Luxembourg prime minister, but if I did that, people would remind me that I am no longer PM, I am now EU commission president," Juncker said.