Washington: The United States and European Union expressed cautious optimism on Friday over prospects that Iran may be willing to engage major powers in new talks, but underscored any new negotiations must be sustained and focus on the nuclear issue.   

Secretary of State Hillary Clinton and European Union foreign policy chief Catherine Ashton told reporters that Iran's recent letter to Ashton might mark a step forward.   

"We think this is an important step and we welcome the letter," Clinton said in a joint meeting with Ashton. She stressed that the major powers were still reviewing their formal response to Tehran's offer.   

 Ashton, who handles contact with Iran on behalf of the "P5+1" group comprised of the United States, Britain, France, Russia, China and Germany, said the letter showed "a potential possibility that Iran may be ready to start talks."   

Iran's letter to Ashton, which was obtained by Reuters on Thursday, proposed resuming the stalled talks and said Tehran would have "new initiatives" to bring to the table. .   

But the brief letter, which responded to a letter Ashton sent to her Iranian counterpart in October, offered no specific proposals, leaving a question mark over Tehran's willingness to enter substantive negotiations on its nuclear work.   

Clinton, however, said the Iranian letter "appeared to acknowledge and accept" the western countries' longstanding condition that any talks begin with a discussion of its nuclear program.   

"We must be assured that, if we make a decision to go forward, we see a sustained effort by Iran to come to the table, to work until we have reached an outcome that has Iran coming back into compliance with their international obligations," Clinton said.   

Real talks Vs buying time
The United States and its allies suspect Iran is seeking to develop nuclear weapons capability under cover of a declared civilian nuclear energy program, and believe Tehran has used talks only as a time-buying tool, not a pathway to agreement.     

Iran says it is enriching uranium only for civilian purposes.   

French Foreign Minister Alain Juppe said on Thursday that a Feb. 20-21 visit to Iran by top U.N. nuclear watchdog officials would help determine whether Tehran was serious about tackling international concerns.     

The U.N. team, led by the International Atomic Energy Agency's chief inspector, will again try to extract Iranian explanations, after three years of stonewalling, for an IAEA investigation driven by intelligence reports that suggest Tehran has researched sophisticated ways to build atomic bombs.   

Following an IAEA report in November that cast new doubts over Iran's nuclear work, the United States and EU adopted sanctions meant to shut down Iran's oil export industry, the world's fifth largest.     

The clampdown on Iranian oil would take full effect in July, and would join an escalating range of U.N. and unilateral sanctions which western officials say are putting unprecedented pressure on Iran's economy.   

 Ashton said the P5+1, which made no headway in its last talks with Iran on the nuclear issue in Istanbul in January 2011, said the letter from chief Iranian negotiator Saeed Jalili contained "no preconditions and a recognition of what we'll be talking about."   

 "The next question really is to look at then where we left off in Istanbul," Ashton said, noting a series of suggested confidence-building measures such as greater scope for inspections.   

"We also said at that time they could come forward with their own ideas about what they wanted to do, so that this was a genuine, open process," Ashton said, saying it was crucial to ensure that any talks, once started, are sustained.   

"Therefore we need to set in train the process whereby we can be clear what it is we need to achieve and what we are expecting from the Iranians and that's what we are in the process of doing right now," she said.