MONTREUX, Switzerland — Negotiations on a potential nuclear deal with Iran resumed Monday, with Secretary of State John Kerry saying the sides are still far apart and warning Israel’s leader that leaks about the talks will hurt their chances for success.

Kerry and Iranian Foreign Minister Mohammed Javad Zarif sat down in the Swiss resort town of Montreux on Lake Geneva for their sixth round of discussions this year. They face an end-of-March target to reach the outline of a deal that would eliminate Iran’s ability to develop nuclear weapons over the length of the agreement in exchange for sanctions relief.

The stakes have been raised by Israeli Prime Minister Benjamin Netanyahu, who adamantly opposes the deal and will address a joint meeting of Congress on Tuesday about the dangers it poses to Israel and others.

In Geneva, Kerry defended Israel at the U.N. Human Rights Council, pledging that the United States would continue to oppose anti-Israel action and bias at the United Nations and elsewhere.

Pushing back against Netanyahu’s expected speech, Kerry maintained that the United States would never allow Iran to get the bomb. And, in a jab at Netanyahu, he insisted that critics of the emerging deal were wrong and should not publicize details of the as-yet-incomplete agreement as senior Israeli officials have said the prime minister will do.

“Right now, no deal exists, no partial deal exists,” Kerry told a news conference in Geneva. “And unless Iran is able to make the difficult decisions that will be required, there won’t be a deal. Nothing is agreed until everything is agreed. That is the standard by which this negotiation is taking place, and anyone who tells you otherwise is simply misinformed.”

But he suggested that revealing details of any preliminary understandings with Iran could be fatal to the goal of a diplomatic resolution to fears of a nuclear-armed Iran.