Iran has been engaged for a year in negotiations with France, Germany, Britain, Russia and China directly, and the United States indirectly in the Austrian capital of Vienna to revive the 2015 nuclear agreement.
Return to 2015 nuclear deal remains uncertain and is not imminent, Washington says, despite optimism in recent weeks that indirect talks between Iran and US could soon produce an agreement.
Extra demands made by Washington contradict its position on reaching an agreement swiftly, says Iranian FM Hossein Amirabdollahian, without specifying the demands.
The visit by International Atomic Energy Agency head Rafael Grossi is seen as critical to clinching a return to the nuclear agreement, and comes parallel to negotiations in the Austrian capital.
The UN nuclear watchdog's Director General Rafael Mariano Grossi will meet with senior Iranian officials in Tehran to discuss the stockpile that the group says can easily be used for atomic weapons.
Iran's Foreign Ministry spokesman has said that an appropriate pathway is required in the areas of lifting sanctions, commitment guarantees from the US and accusations levelled against the nuclear programme.
Senior US State Department official says negotiators have made significant progress regarding Iran nuclear deal and it's in a better position, but added some serious issues still remains.
"Being near the finish line is no guarantee to crossing it" and sealing an accord, Iran's chief negotiator Ali Bagheri Kani says.
State Department reports "substantial progress" in talks at Vienna but also warns "anything much beyond that would put the possibility of return to the  deal at grave risk."
The missile is propelled by solid fuel and the preparation time needed for its launch has been cut to one sixth of what is normally required.
The talks on Iran nuclear accord paused on January 28 as top negotiators returned to their respective capitals for consultations.
In a first, Tehran says it is open to holding direct parleys with US to salvage the 2015 nuclear deal, with Washington reiterating its preparedness "to meet directly" with Iranians.
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