As the hours counted down Wednesday to a crucial summit about the future of the euro zone, prospects for a quick agreement remained uncertain.

European leaders will meet Thursday and Friday to try to work out a deal to ease the financial crisis. But a senior German official said Wednesday that he was not sure whether other countries were fully on board with a Franco-German proposal, even as France’s finance minister vowed that German and French leaders would keep negotiating until they had something to show the world.

“We have the impression that a range of stakeholders have not yet understood the seriousness of the situation,” the German official told reporters in Berlin, speaking on condition of anonymity at a background briefing before he departed for the Brussels summit. He said he was “pessimistic” that the summit could meet its goal of reaching an agreement by Friday.

If the summit fails to live up to high expectations, borrowing costs could soar for troubled countries such as Italy and Spain, potentially exacerbating Europe’s problems. They have eased in recent days as investors bank on a clear plan from the meeting.

The German official’s assessment differed from that of French Finance Minister Francois Baroin, who said in a television interview Wednesday that German Chancellor Angela Merkel and French President Nicolas Sarkozy would not “leave the negotiating table of this summit until there is a powerful deal.”

Germany and France have proposed rules that would limit countries’ deficits to 3 percent of gross domestic product and their overall debt to 60 percent of GDP. Part of the disagreement on Wednesday appears to be based on how to put the rules in place. Germany wants a full-blown treaty, a time-consuming process that requires country-by-country ratification, and in some places may even force public referendums with uncertain outcomes. Previous treaty changes have taken years.

European Council President Herman Van Rompuy and others have suggested much speedier changes to the ground rules of the European Union — revisions that could be made without consulting each national parliament individually. But Germany feels this would not work.

“We’ll get the crucial result before Christmas . . . even if it takes several rounds of negotiations,” the German official said. He added that Germany would be open to a treaty comprised of just the 17 countries using the euro, and any others who wanted to join, if it appears impossible to get all 27 countries in the European Union to agree.

Germany’s renewed pressure could be in part an exercise in brinksmanship, intended to put pressure on dawdling countries and officials to get on board in advance of the summit. As recently as a few days ago, German officials sounded privately optimistic about the prospects of an initial agreement this weekend, followed by negotiations about the specifics. Sarkozy said Monday those talks could last until March.

Merkel last week sought to downplay expectations for the summit, calling Europe’s efforts to ease the crisis a “marathon.”

Ireland and the Netherlands have been cautious about a new treaty, because voters there have proved uninterested in signing on to new European Union proposals in the past. And Britain, which does not use the euro, has expressed fear that a more tightly bound euro group would cause the European Union to fall apart from the divisions it would create.

British Prime Minister David Cameron has sought to use approval of the euro-zone changes as leverage to take back more power for British domestic decisions from the European Union.

If Europe’s leaders can agree on a sufficiently far-reaching overhaul of their economic rules in the coming days, the European Central Bank has hinted that it would take stronger action to support the borrowing of troubled European countries, a step that analysts say would quickly ease the worst crunch of the crisis. The bank’s governing council is widely expected to announce an interest rate cut at a meeting on Thursday.

In a sign of American concern over the future of the euro zone, Treasury Secretary Timothy F. Geithner is visiting European officials this week. Tuesday he met with the head of the European Central Bank, Mario Draghi, and with German Finance Minister Wolfgang Schaeuble. On Wednesday, he told reporters after a meeting in Paris with Baroin that he felt Europe was on the right track.

“I have a lot of confidence in what the president of France and the minister are doing, working with Germany to build a stronger Europe,” Geithner said.

Special correspondent Eva Schroeder contributed to this report.