* Six powers "show a way" to easing sanctions
* Iran foreign minister says talks reach "milestone"
* April meeting will be key for hearing Iranian position
* Diplomat denies six powers softened position on Fordow
Nuclear talks between Iran and world powers this week were more constructive and positive than in the past, but Iran's willingness to negotiate seriously will not become clear until an April meeting, a senior Western diplomat said on Thursday.
The diplomat was more upbeat about the talks in Kazakhstan than other Western officials have been, suggesting there could be a chance of diplomatic progress in the long standoff over Iran's nuclear activities.
"This was more constructive and more positive than previous meetings because they were really focussing on the proposal on the table," said the diplomat, speaking on condition of anonymity.
Iran's Foreign Minister Ali Akbar Salehi struck an upbeat note about the talks, saying they had reached "a turning point" this week and suggesting a breakthrough was within reach.
"I call it a milestone. It is a turning point in the negotiations," Salehi told Austrian broadcaster ORF during a visit to Vienna for a United Nations conference.
"We are heading for goals that will be satisfactory for both sides. I am very optimistic and hopeful," he said, according to a German translation of remarks he made in English.
Years of on-off talks between Iran and the six powers have produced no breakthrough in the dispute over the nuclear programme, which Iran says is peaceful but that Western powers suspect is aimed at developing a nuclear bomb capability.
Iran has faced tightening international sanctions over its nuclear programme and Israel has strongly hinted it might attack Iran if diplomacy and sanctions fail.
At the latest talks, the six powers offered modest sanctions relief in return for Iran curbing its most sensitive nuclear work.
"We show a way into the easing of sanctions. We don't give away the crown jewels in the first step," the diplomat said.
The two sides agreed to hold expert-level talks in Istanbul on March 18 to discuss the powers' proposals, and to return to Almaty for political discussions on April 5-6.
The March meeting will be a chance for experts to explain in detail what the six powers' offer means, the senior Western diplomat said, adding that the April meeting would be key.
"This will be the important meeting. We'll see if they are willing to engage seriously on the package," the diplomat said.
Western officials said the six powers' offer included easing a ban on trade in gold and other precious metals and relaxation of an import embargo on Iranian petrochemical products.
In exchange, a senior U.S official said, Iran would among other things have to suspend uranium enrichment to a fissile concentration of 20 percent at its Fordow underground facility and "constrain the ability to quickly resume operations there".
The U.S. official did not term what was being asked of Iran as a "shutdown" of the plant, as Western diplomats had said in previous meetings with Iran last year.
The senior Western diplomat denied the six powers had softened their position on Fordow, but conceded: "We may have softened our terminology."
The diplomat sketched out a step-by-step approach, saying the six powers' proposals offered Iran the prospect of further steps in return for Iranian actions beyond a first confidence-building step. "There has to be a clear sequencing," the diplomat said, without giving details.
Iran's chief nuclear negotiator Saeed Jalili said on Wednesday the six powers had tried to "get closer to our viewpoint", which he said was positive.