Iran has not formally responded to a U.S. offer last week to talk with Iran in a joint meeting with the countries that negotiated the deal.

Asked at a news briefing whether there was an expiration date on the offer, Price said Iran’s moves away from compliance with the 2015 agreement’s restrictions on its nuclear activities made the issue an “urgent challenge” for the United States.

“Our patience is not unlimited, but we do believe, and the president has been clear on this … that the most effective way to ensure Iran could never acquire a nuclear weapon was through diplomacy,” Price said.