Eurogroup head Jeroen Dijsselbloem said Monday eurozone finance ministers had not been able to find a solution to provide short-term funding to Greece while it negotiates another huge debt rescue programme.
"This is very complex, we looked at a number of possibilities. We have not yet found the 'golden key' to solve this issue," Dijsselbloem said just hours after eurozone leaders agreed to offer Greece a new bailout in return for draconian reform commitments.
The mooted, 3-year, 86-billion-euro programme could take months to negotiate but in the meantime Greece must repay billions to creditors, including the European Central Bank on July 20.
On June 30, Greece failed to make a payment to the International Monetary Fund, becoming the first developed economy to default on an IMF loan and sparking a crisis only partially resolved by the latest eurozone rescue offer.
Finnish Finance Minister Alexander Stubb, one of Greece's harshest critics, said he expected the discussions on a bridging loan to prove hard going.
"I foresee those negotiations being very difficult because I don't see many countries having a mandate to give money without any conditions," Stubb said as he arrived for the eurozone finance ministers meeting in Brussels earlier Monday.
"We must find short-term solutions in relation to Greece's financial deadlines," said EU Economic Affairs Commissioner Pierre Moscovici, who has been among the most sympathetic to Greece's plight.
Dijsselbloem said Monday's talks were "intense" as ministers repeated the need for Greece to live up to its reform promises and restore trust with its eurozone peers.
He said officials would continue working, with ministers to talk again Wednesday, or early Thursday, in a teleconference to discuss progress.
The Greek government of Prime Minister Alexis Tsipras must pass a host of reform legislation no later than Wednesday as a first step towards getting the bailout.