The United States’ patience with Iran on returning to discussions over the 2015 nuclear deal is “not unlimited,” State Department spokesman Ned Price said on Wednesday. Iran has not formally responded to a US 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, Mr 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,” Mr Price said. In the week since Washington offered to talk with Tehran about reviving the 2015 nuclear deal, Iran has curbed UN monitoring, threatened to boost uranium enrichment and its suspected proxies have twice rocketed Iraqi bases with US soldiers. In return, the United States and three allies, Britain, France and Germany, have responded with a studied calm. The response – or lack of one – reflects a desire not to disrupt the diplomatic overture in hopes Iran will return to the table and, if not, that the pressure of US sanctions will keep taking its toll, US and European officials said. Iran has repeatedly demanded the United States first ease the sanctions imposed after former president Donald Trump abandoned the deal in 2018. It would then wind down its own violations of the pact, which began a year after Mr Trump's withdrawal. On Wednesday, President Joe Biden's CIA nominee William Burns said it was important that the US to "everything" it can to prevent Iran from acquiring nuclear weapons. “Challenges posed by Iranian behaviour are not ones we can afford to ignore, across the board: ballistic missile development as well as subversive and destabilising actions in the Middle East and human rights abuses inside Iran as well,” Mr Burns, who conducted covert negotiations with Iran during the Bush and Obama administrations, said in his confirmation hearing.