"We have made a lot of progress with the main point of the treaty, but we are not there yet where we want to be from a Swiss point of view. The demands of the Americans and the expectations of the Swiss are not yet in line and it will certainly need more talks," Widmer-Schlumpf said.

Earlier this month, the upper house of the Swiss parliament backed an amendment that would allow Switzerland to compel its banks to hand over American client data, even if authorities don't already know the client names; the lower house still has to approve it.

The amendment covers an existing tax treaty between the United States and Switzerland in which the Alpine country has agreed to hand over client data but generally only if the U.S. side already knows the client's identity.