The Greek debt crisis could spread to at least five other European countries — including Belgium or even Italy — if it is not cautiously managed, the head of the euro group warned Saturday.

Jean-Claude Juncker told the German daily Sueddeutsche Zeitung that nudging private creditors to contribute to the next Greek bailout package could be considered a "default" by rating agencies — and that would have extreme consequences for Europe as a whole.

Juncker, the prime minister of Luxembourg who also chairs the group of 17 eurozone finance ministers, was quoted as saying that a Greek bankruptcy "could prove contagious for Portugal and Ireland, and then also for Belgium and Italy because of their high debt burden, even before Spain."

"We are playing with the fire," he told the paper.

On Friday, German Chancellor Angela Merkel softened her government's insistence on having private creditors share a part of the burden following a meeting with French President Nicolas Sarkozy in Berlin, with both leaders agreeing that the participation should be "voluntary."

Rating agencies as well as the European Central Bank have warned that forcing bond holders to accept losses or give Greece extra years to repay its debt would likely be considered a partial default by Greece that could spread panic on financial markets.

Juncker stressed that a debt restructuring would ravage Greek banks, which hold a large amount of their country's debt — some €80 billion ($114 billion) — ultimately requiring yet another bailout for Greek banks.

"Everything is becoming yet more expensive because we are including private creditors due to domestic political considerations in Germany," Sueddeutsche Zeitung quoted him as saying.

Merkel's parliamentary majority recently agreed to the latest round of financial assistance for Greece while insisting that private creditors share any potential losses.

The leader of Europe's biggest economy restated that position on Saturday.

"We again have to show solidarity and also have to include private creditors," Merkel told fellow members of her conservative party in Berlin.

European officials will hold talks with the private sector — mostly banks, insurance companies and pension funds — and they will make a "substantial contribution" to a second bailout for Greece, Merkel said, according to the news agency DAPD.

But neither Merkel nor German Finance Minister Wolfgang Schaeuble have given an estimate of the private sector contribution they hope to achieve.

Schaeuble, meanwhile, told the German daily Boersen Zeitung the contribution must be "substantial, measurable and reliable" so that the burden of a new Greek aid package does not fall solely on taxpayers.

Heavily indebted Greece received a €110 billion ($157 billion) bailout last year, but the country is expected to need a second bailout as its borrowing costs remain prohibitively high, making a return to the bond markets impossible.

European finance ministers are meeting Sunday and Monday to discuss the crisis, and it will certainly be a key item at a summit of the EU's 27 leaders later in the week.