WASHINGTON — European officials Sunday agreed to make as much as $40 billion in below market rate loans available to Greece, hoping to allay fears about a possible economic collapse in the troubled country.

In an emergency videoconference among finance ministers, the 16 nations that use the euro added critical detail to the largely conceptual rescue plan they had approved last month.

Without those details, markets remained skeptical that Greece could raise the money needed for its government to continue operating amid high deficits and a weak economy.

The result was a historic spike in the interest rates paid by Greece, waning interest in its bond issues, and rising concern about a possible default by one of the “eurozone” countries.

The loans would be made available at 5 percent — compared to the more than 7.5 percent rates Greece’s 10-year bonds touched last week. Additional loans would be made available by the International Monetary Fund, although the amount and terms of that assistance would be worked out when — and if — Greece asks for help.

So far, the nation has said it wants to try to manage its problems on its own and has not asked for assistance, hoping that the promise of it alone will help it restructure its debts at a reasonable cost. Sunday’s statement could prove important toward that end: if investors know that Greece can now tap money at 5 percent if it needs to, market interest rates should moderate.

European leaders said the plan is an important acknowledgement that the 16 eurozone nations will stand behind the common currency, as well as behind the concept of a more integrated European economy. The crisis in Greece had raised doubts about the survival of the euro, creating a situation where more economically healthy nations like Germany are now having to take responsibility for the problems of others, and constricting some of the options that would have been available to Greece if it had its own currency.

The plan “shows that the euro area is serious in doing what is necessary to secure financial stability,” European Commission president Jose Manuel Barroso said in a written statement. “Europe has shown that responsibility and solidarity can go together.”

The potential fallout from Greece’s troubles run beyond the country itself, which is already facing tax hikes, limits on retirement pay, and other measures to bring down government spending. Some analysts are concerned that the “contagion” of a Greek default would put pressure on other European nations with large amounts of government debt.