Iran and six world powers have ended high-level nuclear talks in Vienna after four days.

Experts from Iran, the United States, China, Britain, France, Germany, Russia, the European Union and the U.N. International Atomic Energy Agency (IAEA) attended the talks.

They met to discuss how to implement last month’s breakthrough deal for Tehran to curb its nuclear programme in return for easing of sanctions.

But they need more time to work out technical steps.

A European Union spokesman said there would be consultations and talks are expected to continue soon. He gave no details of specific issues, however translating the interim deal into an action plan is said to be complicated.

The deal is seen as a first step to resolving a standoff over suspicions Iran might be pursuing a nuclear weapons capability, something Tehran denies.

The decision to halt the talks came hours after Washington blacklisted a dozen overseas companies and individuals for evading crippling US sanctions on Iran’s oil and banking sectors.