Angela Merkel has vowed to create a "fiscal union" across the eurozone with wide-ranging powers to avert catastrophe, saying the process was already under way as part of the "marathon" effort to solve the European debt crisis.

The German chancellor said she was determined to push for treaty changes at next week's EU summit, and again reiterated German opposition to eurobonds.

Despite criticism that her indecisiveness is accelerating the possibility of a collapse of the single currency, Merkel preferred to focus on what had been agreed in recent months. "It's no exaggeration to say that we have achieved an incredible amount," she said. "In Europe we are now arguing and wrestling over the fine print, not about the plan as a whole," she added.

"Anyone who, a few months ago, had said that at the end of the year 2011 we would have taken very serious and concrete steps towards a European stability union, a European fiscal union with powers of enforcement, would have been considered crazy," she said. "Now, this is exactly what's on the agenda. We're almost there. Of course, there are difficulties to be overcome. But the necessity of such action is widely recognised. We're not just talking about a fiscal union but starting to create one. I believe you can't overestimate the importance of this step."

She said she wanted to "change the basis of [European] co-operation – through, for example, treaty change – in order to create a fiscal union with powers of enforcement, at least for those countries in the eurozone."

She said: "I want to stress that we are going to Brussels with the goal of treaty changes [rather than proposing wholesale new treaties] because we want to avoid a split between eurozone member states and non-eurozone members. No one should wish for such a split," she said, adding: "The eurozone should remain open for those who want to join it."