European leaders are preparing to meet in Brussels Thursday for the start of a summit that could determine the fate of the euro.

Some officials have described the European summit as a moment of reckoning for the common currency, under siege by the continent’s ballooning debt crisis. But as the opening of the meeting draws closer, optimism is waning.

German Chancellor Angela Merkel and French President Nicolas Sarkozy are calling for all 17 nations using the euro to adhere to balanced budgets and enforce strict penalties on violators. Their plan would also create a unified corporate tax rate and a new financial transaction tax.

Mrs. Merkel and Mr. Sarkozy are first trying rally support for their plan during a meeting of the center-right European People’s Party in the French city of Marseille, before heading to Brussels. But senior officials said late Wednesday that final agreement on a deal could be weeks away.

One German official told the Associated Press that it might be Christmas before a deal is reached. Another senior official was quoted as saying that some countries “have not understood the seriousness of the situation.

Luxembourg Prime Minister Jean-Claude Juncker, who chairs the group of eurozone ministers, told French radio Thursday the goal is to get all 27 European Union members on board. He said if that was not possible, officials would pursue a new treaty for all 17 countries using the euro.

Fears that Europe’s debt crisis could spark additional problems have sent jitters through the global financial markets and have prompted warnings from several top credit rating agencies.

Standard & Poors Wednesday put the the EU’s long-term rating on a credit watch, suggesting a downgrade could be imminent.

U.S. President Barack Obama has repeatedly urged European leaders to do what is necessary to fix the crisis as soon as possible and spoke with German Chancellor Merkel by telephone late Wednesday.

The White House said President Obama and the German chancellor agreed on the need to find a lasting and credible solution for the eurozone crisis.

But Britain, which does not use the common euro currency, has indicated that it may not be willing to sign on to the new EU treaty provisions. Prime Minister David Cameron said earlier this week that he will defend Britain’s interest at the summit in Brussels.