The US State Department on Wednesday maintained that the decision to send a top US diplomat to talks with an Iranian negotiator was not a change in policy.

William Burns, America's third highest-ranking diplomat, will attend talks with the Iranian envoy, Saeed Jalili, in Switzerland on Saturday.

At the daily State Department briefing, spokesman Sean McCormack suggested that the move was merely a change in tactics.

"All I can go back to is a point that I'm making about the centrality of the condition for realising negotiations. That is unchanged. The substance is unchanged. Is this a new tactic, if you will? Yes. Does it send a signal? Yes. Is the substance any different? No."

The talks are aimed at persuading Iran to halt its controversial uranium enrichment activities. It will be the first time such a high-ranking US official has attended such talks.

The two countries have not had diplomatic relations since the hostage crisis at the US Embassy in Tehran after the 1979 Islamic revolution in Iran.

In related news, Iran's supreme leader Ayatollah Ali Khamenei said Wednesday that Tehran has a clear "red line" in its talks with major powers over its controversial nuclear program.