French Foreign Minister Bernard Kouchner said on Thursday big powers were waiting for specific proposals from Iran over its disputed nuclear programme and a new U.S. presence at a pending Geneva round should be helpful.

The United States said on Wednesday it was sending an envoy to join nuclear talks with Iran for the first time to underline to the Islamic Republic and others that Washington desired a diplomatic solution to the volatile impasse.

Kouchner welcomed U.S. participation at Saturday's Geneva talks along with British, French, German, Russian and Chinese envoys, aimed at probing for any Iranian flexibility on terms for obtaining trade benefits if it suspends uranium enrichment.

"This is not only a good initiative, but our U.S. friends are joining the dialogue. It's a very interesting, a new attitude, an additional asset, no doubt," Kouchner told reporters outside a European security meeting in Vienna.

He said Iran's dismissive response so far to a revised packet of incentives offered by the powers if it shelves nuclear activity that could yield atom bombs was "not surprising, a bit sad," but its readiness to discuss it further was encouraging.

"We are waiting for an opening. I talked to (Iranian Foreign Minister Manouchehr)Mottaki and he was open, but open to what? That is always the case. We talk and talk with the Iranians, but it's always disillusion," said Kouchner, speaking in English.

He said Tehran was still not addressing "the core of the subject" - an enrichment suspension, or an interim freeze on steps to expand the proliferation-prone activity, to get preliminary negotiations going. "But they accepted dialogue so let's start dialogue."