David Ignatius writes: President Obama has signaled Iran that the United States would accept an Iranian civilian nuclear program if Supreme Leader Ali Khamenei can back up his recent public claim that his nation “will never pursue nuclear weapons.”

This verbal message was sent through Turkish Prime Minister Recep Tayyip Erdogan, who visited Khamenei last week. A few days before traveling to Iran, Erdogan had held a two-hour meeting with Obama in Seoul, in which they discussed what Erdogan would tell the ayatollah about the nuclear issue and Syria.

Obama advised Erdogan that the Iranians should realize that time is running out for a peaceful settlement and that Tehran should take advantage of the current window for negotiations. Obama didn’t specify whether Iran would be allowed to enrich uranium domestically as part of the civilian program the United States would endorse. That delicate issue evidently would be left for the negotiations that are supposed to start April 13, at a venue yet to be decided.

Erdogan is said to have replied that he would convey Obama’s views to Khamenei, and it’s believed he did so when he met the Iranian leader on Thursday. Erdogan also met President Mahmoud Ahmadinejad and other senior Iranian officials during his visit.

The statement highlighted by Obama as a potential starting point was made on state television in February. Khamenei said: “The Iranian nation has never pursued and will never pursue nuclear weapons. . . . Iran is not after nuclear weapons because the Islamic Republic, logically, religiously and theoretically, considers the possession of nuclear weapons a grave sin and believes the proliferation of such weapons is senseless, destructive and dangerous.”

The challenge for negotiators is whether it’s possible to turn Khamenei’s public rhetoric into a serious and verifiable commitment not to build a bomb. When Obama cited this statement to Erdogan as something to build on, the Turkish leader is said to have nodded in agreement.

But the diplomatic path still seems blocked, judging by recent haggling over the meeting place for negotiations. Istanbul was expected to be the venue, but the Iranians last weekend balked and suggested instead that negotiators meet in Iraq or China. U.S. officials see this foot-dragging as a sign that the Iranian leadership is still struggling to frame its negotiating position.

The Erdogan back channel to Iran is the most dramatic evidence yet of the close relationship Obama has forged with the Turkish leader. Erdogan, who heads an Islamist party that is often cited as a model by Muslim democrats, has been a key U.S. partner in handling Syria and other crises flowing from the Arab Spring uprisings.

Meanwhile, the Los Angeles Times reports: A war of words between Turkish and Iranian leaders intensified Thursday, threatening to delay or even scuttle a new round of talks between Iran and world powers, and raising fresh doubt about whether Tehran will bargain over its disputed nuclear program.

One day after Iranian leaders ruled out talks in Istanbul next week because of Turkey’s position on the Syrian uprising, Turkish Prime Minister Recep Tayyip Erdogan accused Iran of dishonesty.

“It is necessary to act honestly,” Erdogan said at a news conference in Ankara, Turkey, according to Reuters. The Iranians “continue to lose prestige in the world because of a lack of honesty.”

The broadside marked a remarkable turnabout for Erdogan, who has worked hard for years to cultivate ties with Tehran. He has repeatedly taken Iran’s side in its dispute with the West over its nuclear development efforts.