A breakthrough in reviving the 2015 Iran nuclear agreement would depend on the United States, experts said, after Teheran and Washington unsuccessfully ironed out their differences in Qatar where informal talks to salvage the pact ended on Wednesday with little progress.

Dina Yulianti Sulaeman, director of the Indonesia Center for Middle East Studies, said the latest development is "as usual "because the US was also forcing Iran to comply with its wishes.

"Iran's position is very clear, which is to return to the JCPOA(Joint Comprehensive Plan of Action), which was signed in 2015. However, the US continued to insist on adding and reducing its contents, then even unilaterally withdrew from the JCPOA. And when Iran does not want to follow the will of the US, the US even imposes sanctions on Iran," Sulaeman said.

"So, what is needed for successful negotiation is the goodwill from the US, namely respecting the JCPOA which is the result of years of hard work from P5+1 diplomats."

Iran signed the landmark deal in 2015 with the five permanent members of the United Nations Security Council－China, France, Russia, the United Kingdom and the US－plus Germany, known as the P5+1. Since April last year, eight rounds of talks have been held in Vienna, but they have stalled since mid-March due to major differences between Teheran and Washington.

European Union envoy Enrique Mora tweeted that it was "not yet the progress the EU team, as coordinator, had hoped for". But he also said they would keep working with even greater urgency to restore "a key deal for nonproliferation and regional stability".

Low expectation

In an interview with the Al Mayadeen media network based in the Lebanese capital Beirut, Seyed Mohammad Marandi, an adviser to the Iranian negotiating team to Vienna, said: "There was never any expectation that these negotiations would culminate in an agreement over a couple of days."

Marandi said Teheran does not take US officials' media statements seriously, and that the US should assure guarantees that "history is not repeated".

Kamaruzaman Bin Yusoff, a Middle East political analyst and former professor of Middle Eastern politics at Universiti Teknologi Malaysia, said in his view, the negotiations "have concluded in 2015", but it was the US administration that put everything on hold.

Henelito Sevilla Jr, dean and professor in the Asian Center at the University of the Philippines, said the resumption of nuclear talks between the US and Iran in Qatar "should be embraced positively, not just by two parties but also the international community", as both are crucial actors in the nuclear negotiations.