Iran and world powers completed Wednesday what both sides called a "substantive" round of nuclear talks, with Iran's foreign minister saying he saw "signs" a comprehensive deal will be reached.

"We had substantive and useful discussions covering a set of issues including (uranium) enrichment, the Arak reactor, civil nuclear cooperation and sanctions," EU foreign policy chief and the powers' lead negotiator Catherine Ashton said in Vienna.

Iran's Foreign Minister Mohammad Javad Zarif, who read the same statement in Farsi, also said he saw "signs" that the two sides would reach their goal of transforming by July an interim deal from November into a lasting agreement, with the next round due from April 7-9.

"There are signs that an understanding is possible that respects the rights of the Iranian nation," Iranian media quoted Zarif as saying in Vienna.

The next round of talks, part of a series aimed at resolving for good the decade-old standoff and silencing talk of war, will again be in Vienna, preceded by an experts' meeting from April 3-5.

A senior US administration official in Vienna said that the latest two days of talks saw the parties "really get down to business and into the details", with the meeting "professional, respectful and intense".

Negotiators entered into "the substance of key issues more than we have ever previously," the official said, making "progress in identifying where the gaps exist and working to bridge those gaps".

The official added however that "hard work" remained to be done, and a senior Iranian negotiator said earlier Wednesday that more time was needed before starting to draft a final deal.

"It is too early to enter into negotiations for drafting a text for a final agreement," negotiator Abbas Araqchi was quoted as saying by ISNA news agency.

Under November's interim agreement, Iran froze key parts of its nuclear programme in return for minor sanctions relief and a promise of no new sanctions for six months.

Although it could be extended, the deal is due to expire on July 20.

The six powers -- the United States, China, Russia, Britain, France and Germany -- now want Iran to reduce permanently, or at least for a long time, the scope of its nuclear activities in order to make it extremely difficult to develop nuclear weapons.

This would likely include Iran slashing the number of centrifuges enriching uranium -- which can be used for peaceful purposes but also in a bomb, if highly purified -- and allowing tougher UN inspections.

- Hard sell to hardliners -

But even though in return Iran would see sanctions lifted, it remains uncertain whether ultra-conservative elements in Tehran around supreme leader Ayatollah Ali Khamenei would accept such limitations.

Any deal that leaves some of Iran's nuclear infrastructure intact would also be a hard sell to sceptical US lawmakers and to Israel, the Middle East's sole if undeclared nuclear power.

On Tuesday US House and Senate members sent wish lists to President Barack Obama urging him to stay tough in the negotiations and stick to "core principles".

So far, despite disagreements over Syria and other issues, the powers have been united over Iran, but events in Ukraine in recent weeks have precipitated the worst crisis in East-West relations since the Cold War.

Iranian and Western officials said however that the crisis has had no effect on the Iran talks.

"People stayed very focused on the job in hand," the US official said. "I continue to hope that ongoing events in Ukraine ... will not change this."

Even before the Ukraine crisis erupted, Moscow was reported to be discussing a major deal with Tehran whereby Russia would get Iranian oil in exchange for money, goods and help in building new nuclear reactors.

This would undermine Washington's efforts to cut off Iran's main source of revenue -- a strategy which the US credits with forcing Tehran to the negotiating table in the first place.