Iran and six world powers are starting the first round of talks on a long-term agreement on Tehran's controversial nuclear program, BBC News reports.

The talks in Vienna aim to build on November's interim deal, in which Iran agreed to curb uranium enrichment in return for partial sanctions relief.

Ahead of the meeting, both Iran and the U.S. downplayed hopes of a quick breakthrough.

The talks between Iran and Britain, China, France, Germany, Russia and the U.S. are due to open Tuesday, Feb 18, at 11:00 (10:00 GMT) in the Austrian capital.

The West wants Iran to sharply scale back its sensitive nuclear activities to ensure that it cannot quickly assemble an atomic bomb. But Tehran says its nuclear work is purely peaceful and will continue. It also wants an end to the sanctions that have battered its economy.

It took months of very hard bargaining for the two sides to agree to negotiate a final deal. The talk will be a first test of that resolve, the BBC says.

On Monday, Iran's Supreme Leader Ayatollah Ali Khamenei said the Vienna talks "will not lead anywhere", in a statement posted on his website. But he added: "What our officials started will continue. We will not renege. I have no opposition."

Meanwhile, a senior U.S. administration official, according to Reuters, also acknowledged that it would be a "complicated, difficult and lengthy process".

The two sides have until July 19 to find a comprehensive solution.