TEHRAN/WASHINGTON (Reuters) - Iran delivered a letter to world powers on Tuesday but gave no concrete reply to a demand to freeze its nuclear activity, a defiant step the United States said amounted to “obfuscation” and could lead to more sanctions.

Iran's President Mahmoud Ahmadinejad speaks during 15th Ministerial Conference of the Non-Aligned Movement in Tehran, July 29, 2008. REUTERS/Raheb Homavandi

Iran handed the letter to European Union foreign policy chief Javier Solana in response to an offer in June by major powers that they would refrain pursuing more U.N. penalties if Iran froze expansion of its nuclear work.

Extracts of the one-page letter obtained by Reuters showed Iran gave no firm reply to the offer but instead promised a “clear response” at an unspecified date.

“Iran is ready to provide a ‘clear response’ to your proposal at the earliest possibility, while simultaneously expecting to receive your ‘clear response’ to our questions and ambiguities as well,” the letter said.

“Such mutual clarification can pave the way for a speedy and transparent negotiating process with bright prospect.”

The major powers say they fear Tehran wants to build an atomic bomb. But Iran, the world’s fourth-largest oil producer, insists it is only seeking to master nuclear technology to generate electricity.

“It is more of the same from the Iranians -- obfuscation and delays,” said one U.S. official, who asked not to be named because he was not authorized to talk about the letter. “It was not the type of response the international community was looking for.”

U.S. State Department spokesman Gonzalo Gallegos warned of “additional measures” against Iran and said the United States would join a conference call with other senior officials from China, Russia, France, Germany and Britain on Wednesday when the major powers would decide how to proceed.

Tehran has repeatedly refused to halt its atomic work, prompting the U.N. Security Council to impose three rounds of penalties on Iran since 2006. The United States also maintains its own sanctions against Iran.

Diplomats cautioned it would be difficult to pass a fourth round of Security Council sanctions against Iran because of reluctance from Russia and China, as well as Germany.

‘ABSOLUTELY NOTHING’

One Western official who had seen the letter said it added “absolutely nothing” and that Tehran made no concrete proposals to resolve the impasse.

The official said the letter also failed to provide any real response to the offer from the major powers of trade, financial and diplomatic incentives in exchange for an Iranian freeze of its uranium enrichment activities.

An Iranian official had also told Reuters the letter did not address the demands by world powers.

“The letter handed over is not an answer to the offered package. The letter does not mention the freeze-for-freeze issue,” the official said.

The freeze proposal was seen as a step to full negotiations. But the Iranian official said the idea also had not been raised in telephone talks on Monday between Solana and Iran’s chief nuclear negotiator, Saeed Jalili.

“During the call, Jalili expressed his readiness to start formal talks,” the official said, adding he expected further contact between Solana and Jalili in the next few days.

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The six powers have said formal talks on the incentives can start only once Iran suspends uranium enrichment, the part of the program that most worries the West because it has military and civilian uses.

In another development, a U.N. nuclear watchdog official will go to Iran on Thursday. The International Atomic Energy Agency declined to specify the purpose of the visit by Olli Heinonen, its deputy director overseeing inspections of Iran’s nuclear program.

The head of Iran’s Revolutionary Guards said on Monday the country could easily close the Strait of Hormuz, a key Gulf shipping route, if it were attacked over its nuclear program -- prompting a warning from the United States.

“Shutting down the straits and closing off the Persian Gulf would be a sort of a self-defeating exercise,” Pentagon spokesman Geoff Morrell said on Tuesday.

“I don’t think it’s in Iran’s interest,” he told reporters. “They have a very weak economy at this point, which depends almost entirely on their oil revenue.”