Washington has indicated in the past that it’s prepared to talk directly with Iran, and talks involving all five permanent members of the U.N. Security Council plus Germany have made little headway. Several rounds of internationoal sanctions have cut into Iran’s oil sales and financial transactions.

Last month Iran, in a defiant move ahead of a new round of talks expected soon with the six powers, announced plans to vastly increase its pace of uranium enrichment. That can be used to make both reactor fuel and the fissile core of warheads.

Biden told an international security conference that “there is still time, there is still space for diplomacy backed by pressure to succeed.” He did not specify any timeframe.

He insisted that “the ball is in the government of Iran’s court” to show that it’s negotiating in good faith.