European Central Bank executives struggled last week to parry European Union plans to aid Greece via potentially risky schemes that could see Athens declared in default on its sovereign debt. ECB president Jean-Claude Trichet, vice president Vitor Constancio and chief economist Juergen Stark argued that any moves that obliged private investors to take part in a Greek bail-out were fraught with danger. Stark called Friday in Frankfurt for an end to what he said was a \"really fruitless discussion\" on private sector involvement. \"The real issue is the (reform) programme for Greece and to implement the programme in full,\" he stressed. Trichet warned Thursday of the dangers of a \"selective default\" that could wreak havoc across the 17-nation eurozone. And Constancio added a final note of caution. The idea of a concerted, or orderly reduction on the amount of money Greece owed known as a haircut in financial markets was \"definitely a very dangerous path\", he said. Even a mooted rollover of bonds by private creditors, which the ECB does not intend to do with the Greek debt it owns, would not necessarily resolve the Athens\' debt crisis, the ECB executives said. International ratings agency Moody\'s, for one, said it might still be considered a default if it was not clearly voluntary on the part of creditors. Stark warned that new bonds might not qualify as collateral against ECB loans. Governments and other policy makers shaping the new Greek bailout base efforts on \"the underlying assumption that the ECB would continue to accept\" rolled-over Greek debt as collateral for its liquidity operations, Stark said. But he warned: \"This remains to be seen.\" A refusal would leave the commercial Greek banks, which own much of the country\'s debt, dead in the water for many of them are already dependent on the ECB for funds. Any collapse of the Greek financial sector could threaten countries such as Ireland and Portugal, which are also struggling with heavy debt and have accepted international bail-outs. EU leaders are nevertheless pushing ahead with plans to involve private investors in the next rescue plan for Greece. It has already benefitted from an initial package worth 110 billion euros ($160 billion) backed by the EU and International Monetary Fund. EU president Herman Van Rompuy said Friday that he was confident a new aid package for Greece could be agreed by the end of June. The upcoming EU summit on June 23-24 is shaping up to focus primarily on another rescue for Athens, which is said to need 90 billion euros more. In recent days, attention focused on an initiative likened to a 2009 deal in which investors simply rolled over maturing bonds. But the European Commission now suggests the Greek deal could go even further. \"We have also examined the feasibility of a voluntary debt rescheduling or reprofiling, of course on the condition, extremely important, that this would not create a credit event,\" said Amadeu Altafaj, spokesman for EU economic affairs commissioner Olli Rehn. Greek Finance Minister George Papaconstantinou said a new rescue package should be a combination \"of funds coming from the country\'s planned privatisations, the voluntary participation of the private sector and possible loans\" from eurozone countries. In Berlin, German Finance Minister Wolfgang Schaeuble said eurozone finance ministers would create a working group to find a way to involve private investors in any new aid. A second rescue plan was \"inevitable\", he told German lawmakers. But it should establish \"a fair distribution of risks between the taxpayer and private creditor\", to signal \"that you cannot simply dump the risk on the taxpayer\". Constancio insisted that it was the responsibility of EU governments to \"find the mechanisms that do not create the type of risks that we are concerned about\". Finally, Trichet repeated a call for stronger eurozone economic governance, a long-term project that would require capitals to relinguish some sovereignty to ensure a stronger future. \"We should ourselves \'learn to think (more) continentally\',\" Trichet told a conference of analysts and other ECB observers, quoting Alexander Hamilton, a US founding father who established that country\'s first national bank.