Iran has confirmed that Foreign Minister Mohammad Javad Zarif had discussed prisoner releases with former US ambassador Bill Richardson, after the two countries each released one detainee. A ministry official had said in December that Mr Richardson, a former US envoy to the United Nations, had held talks with Mr Zarif but played down their significance. On Friday, ministry spokesman Abbas Mousavi said they had discussed the release of US navy veteran Michael White "months ago". Mr White, detained in Iran since 2018, was freed on Thursday in exchange for the United States allowing an Iranian-American physician to visit Iran, his lawyer and a US official said. "Foreign Ministry spokesman Mousavi confirmed meeting of Bill Richardson with Zarif over the American detainee," state news agency IRNA said, referring to Mr White. "We have always responded positively to humanitarian efforts that would lead to the release of Iranian hostages in the United States and elsewhere," it quoted Mousavi as saying. Iran's top diplomat on Friday threw the ball back into the US president's court on reaching a new nuclear agreement, after the two countries carried out the prisoner swap. President Donald Trump had voiced hope for progress with Iran a day earlier, after the prisoner swap. "Thank you to Iran, it shows a deal is possible!" Mr Trump had tweeted. Foreign Minister Mohammad Javad Zarif responded to Mr Trump on Twitter on Friday, saying, "We achieved humanitarian swap *despite* your subordinates' efforts". "And we had a deal when you entered office. Iran & other JCPOA participants never left the table," he said, using an acronym for a landmark 2015 nuclear deal. The multilateral accord, known formally as the Joint Comprehensive Plan of Action, gave the Islamic republic relief from international sanctions in return for limits on its nuclear programme. Tensions between Tehran and Washington escalated in 2018 after Trump unilaterally withdrew the US from the agreement and reimposed crippling sanctions on Iran. The other partners to the accord are Britain, China, France, Germany and Russia. "Your advisers - most fired by now - made a dumb bet. Up to you to decide *when* you want to fix it," Mr Zarif added. The door remains open for a wider negotiation with Iran about its nuclear program and other issues, but so far talks have been limited to the prisoner releases, US Special Representative for Iran Brian Hook said on Friday. "(President Donald Trump) has had the door open to diplomacy for many years and in the same time frame, he has met (North Korean leader) Kim Jong Un three times. So, we would like to see the (Iranian) regime meet our diplomacy with diplomacy," he told reporters a day after Iran freed US citizen Michael White. Mr Hook said US-Iranian negotiations have so far not gone beyond discussing the mutual release of prisoners to such issues as Iran's nuclear program. He also said the number of US citizens wrongly detained abroad who will be released will grow, but provided no details on when or where that might happen. The US official reiterated an appeal for Iran to release American citizens Baquer Namazi, his son Siamak Namazi and conservationist Morad Tahbaz. He specifically asked for the latter two to be furloughed from prison on medical grounds. Mr Hook also said Washington plans to stick to its policy of harsh economic sanctions on Tehran in an effort to bring it to the negotiating table on issues such as the nuclear program, saying "timidity and weakness invites more Iranian aggression." "We refuse to play by that rule book. When you play under house rules, the house always wins," Mr Hook said. "So we are going to continue with our policy."