Negotiators for Iran and six world powers on Wednesday adjourned what they described as "substantive and useful" nuclear talks and said they will resume April 7-9 in Vienna, with an agreement expected by June.

The two sides spoke at the end of two days of negotiations focused on curbing Tehran's atomic activities in exchange for full sanctions relief. Their joint statement was read by EU foreign policy chief Catherine Ashton, who speaks for the six countries negotiating with Iran, and Iranian Foreign Minister Mohammed Javad Zarif.

They said the talks addressed Iran's uranium enrichment program, a nearly finished nuclear reactor and the lifting of sanctions on Iran that have been imposed successively over the past decade as Tehran expanded its atomic activities.

A senior U.S. official said that while it would be very difficult to overcome differences between Iran and six world powers over Tehran's uranium enrichment programme, all parties aim to adhere to their 6-month deadline to reach a nuclear deal.

"It's a gap [on enrichment] that's going to take some hard work to get to a place where we can find some agreement," the official said.

The official said the differences over Iran's planned Arak heavy-water reactor, which Western powers fear could yield weapons-grade plutonium, remained similarly wide.

The six want the nearly finished reactor destroyed or converted to a type that produces less plutonium, a material that could be used to make nuclear weapons.

Iran says it is enriching uranium only to fuel reactors, and for scientific and medical purposes. It is expected to fiercely resist demands that it limit this activity. But the six want to curb enrichment because it can also be used to make nuclear weapons.
Iran denies wanting such arms.

The sides began the latest round of talks with Western and Iranian diplomats saying the Ukraine crisis has not complicated their efforts so far.

The meeting was the second in a series that Western governments - the United States, France, Britain and Germany - as well as China and Russia hope will culminate in a broad settlement of the decade-old dispute that threatens to draw the Middle East into a new war.

The talks are steeped in mutual mistrust and years of adversity, and tensions between Moscow and Western capitals over Crimea could further strain diplomacy, because Russia has in the past differed with the West in their approaches toward Tehran.

Iran's top nuclear negotiator Abbas Araghchi said that the crisis in Ukraine - the worst confrontation between the West and the East since the Cold War - had so far had "no impact" on talks with the six nations.

"We also prefer the [powers] to have a unified approach for the sake of negotiations," he told reporters late on Tuesday, noting that the first day of talks was "positive and very good."

A spokesman for Ashton, who coordinates diplomacy with Iran on behalf of the six, said that the powers were working in a "unified fashion.'

In the past, Russia has generally enjoyed warmer ties with the Islamic Republic and suggested Western fears about any nuclear weapons designs by Tehran are overblown.

As in previous meetings, Deputy Foreign Minister Sergei Ryabkov represented Russia at the talks, which were expected to end late in the day.

Iran denies West ern suspicions that its nuclear work has any military aim and wants the West to lift crippling economic sanctions as part of the final deal.

The six powers want Iran to curtail its nuclear program to a point where they would feel secure it could not produce bombs.