Experts have little expectation that an accord, if it can be achieved next week by the negotiating countries — Iran, the United States, France, Britain, Germany, Russia and China — will be the comprehensive and detailed agreement the West has long sought.

And some of the United States’ negotiating partners have already begun to send signals that the immediate goal is a partial understanding that would codify fresh progress while extending the negotiating deadline yet again.

“I’m not optimistic that we can get everything done by Monday,” said Philip Hammond, Britain’s foreign minister, during a trip to Latvia on Wednesday. “But I think if we make some significant movement we may be able to find a way of extending the deadline to allow us to get to the final deal.”

At a news conference after his meeting with Mr. Fabius, Mr. Kerry insisted that his goal for the current round of talks remained resolving the key elements of a comprehensive accord.

“We are not talking about an extension,” Mr. Kerry said. “We are driving towards what we believe is the outline of an agreement that we think we can have.”