A deal is "shaping up" to bring the US back into the nuclear deal with Iran, negotiators said on Wednesday.
"We've made good progress," Enrique Mora, the EU official who chaired the talks between Russia, China, Germany, France, Britain and Iran, tweeted after the talks.
Indirect negotiations between Washington and Tehran have been continuing in the Austrian capital since early April, with the other five countries that are signatories to the deal acting as intermediaries.
"Both on the nuclear side and on the sanctions side, we are now beginning to see the contours of what the final deal could look like. This is different from last time we broke," senior diplomats from France, Germany and Britain said.
"However, success is not guaranteed. There are still some very difficult issues ahead. We do not underestimate the challenges that lay before us."
Iran has also said that the talks were on the right track.
The various sides are due to meet again in the Austrian capital early next week.
Meanwhile, Iranian representatives are holding talks with the International Atomic Energy Agency in Vienna on extending a three-month deal, which expires this week, on the agency's inspections of Iranian nuclear facilities.
The goal is to find a way back to the 2015 nuclear accord, from which former US president Donald Trump withdrew but his successor Joe Biden wants to revive.
For that to happen, the US and Iran must agree on the lifting of sanctions reinstated by Mr Trump and on Tehran's commitment to follow the deal's terms.
When Mr Trump left the agreement, Iran began to abandon the constraints on its production of nuclear material.
Diplomats hope to get the US back on board before Iranian presidential elections on June 18.
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