Economy Minister Luis de Guindos, one of the frontrunners to take over the presidency of the Eurogroup of finance ministers this year, said "unfortunately it is not very likely" that Greece will be able to return to finance markets.
"If Greece cannot gain access to the markets between now and June... we must establish a new agreement with Greece. We can call it an agreement, accord, pact, programme, but it is that," he told reporters at an economic forum in Barcelona.
"It will be a line of credit associated with conditions, as usual," he added.
The European Union has played down talk of any new bailout, insisting that it was still finalising details of the four-month extension of Greece's current EU-IMF programme until June, which was agreed on last week.
Eurozone finance ministers, who are due to meet in Brussels next week, "are not discussing any third aid programme," Simone Boitelle, spokeswoman for Eurogroup chairman Jeroen Dijsselbloem, told AFP on Tuesday.
De Guindos on Monday had fuelled speculation that a third bailout was in the works when he told journalists in Spain that EU officials estimated a rescue package of up to €50 billion ($55 billion) could be needed to keep Greece afloat and avoid a painful exit from the euro.
But German Chancellor Angela Merkel Wednesday played down any talk of a third rescue package, saying the EU was focused on completing Greece's current bailout.
"We are currently doing all that we possibly can to ensure that the second programme be successful," Merkel told a press conference in Brussels after talks with European Commission chief Jean-Claude Juncker.
"The Troika (of Greece's creditors, the EU, ECB and IMF) is going to assess all of this and I think we've got plenty to be getting on with to make this agreement work, that's what I want to focus on."
Juncker added that it was "really premature to be talking in terms of a third programme, that's speculation and I think it's best avoided."
Accepting an additional life-line would amount to a major U-turn for Greece's new hard-left Prime Minister Alexis Tsipras, who swept to power in January on the promise that Athens would end the era of bailouts and their associated austerity requirements.
De Guindos' comments come in the wake of acrimonious swipes between Greece and several eurozone partners - foremost Spain.
The barbs began flying after Tsipras accused Spain and Portugal over the weekend of having sought to cut off EU funding to Greece during negotiations last month on the four-month extension to Athens.
Tsipras claimed the two conservative-ruled countries had sought to undermine Greece's anti-austerity government. Germany labelled Tsipras's comments "unusual foul play," while officials in Spain and Portugal responded with retorts of their own.
Greece had two international bailouts in 2010 and 2012 worth €240 billion - which staved off bankruptcy and kept the country in the euro, but at the cost of punishing economic reforms.