Iran and the United States have made progress on virtually every issue under discussion in indirect meetings over the past two months. But as they began a sixth round of talks Saturday, the Biden administration remained unsure whether they are any closer to final agreement than they were at the beginning. There have been advances “on every issue, every time we meet,” a senior administration official said of the talks aimed at both sides returning to compliance with the nuclear deal they signed in 2015.

Remaining gaps could be closed “in a matter of weeks,” said the official, one of several U.S. and European officials who spoke on the condition of anonymity about the sensitive meetings in Vienna. “But 70 or 80 percent doesn’t matter,” the official said, until basic mistrust between the two sides is overcome and political decisions are made to accept the negotiated results.

Both have stated, in some cases with great specificity, steps they are willing to take toward compliance with the original terms of the nuclear deal. The administration has indicated which of the many U.S. sanctions on Iran it is prepared to lift. Many were imposed, or reimposed, by the Trump administration as part of its “maximum pressure” campaign begun after it withdrew from the nuclear deal in 2018.

“We’ve checked quite a few boxes,” said an official from one of the European countries — Britain, France and Germany — that remain among the original signatories of the deal and are acting as go-betweens for Iran and the United States in the talks. The other signatories, also participating in the talks, are Russia and China.

While unresolved questions of U.S. sanctions and Iranian capabilities remain, “we’re optimistic from a purely mathematical perspective” that “agreement is feasible,” the European official said. But “that’s only part of the answer. The rest is more political.”