Secretary of State Hillary Rodham Clinton and E.U. High Representative Catherine Ashton’s remarks were the first official response to Iran’s negotiation’s proposal, which arrived at E.U. headquarters on Wednesday. (Jacquelyn Martin/AP)

U.S. and European officials on Friday cautiously welcomed a 200-word letter from Iran proposing talks about its nuclear program, a request that appeared to spark a flicker of hope for resuming long-stalled diplomatic engagement with Iranian leaders.

Secretary of State Hillary Rodham Clinton and European Union foreign affairs representative Catherine Ashton expressed wary optimism in their first public response to the Iranian proposal, which arrived at E.U. headquarters Wednesday.

“We think this is an important step, and we welcome the letter,” said Clinton, appearing with Ashton at a joint news conference at the State Department on Friday morning. Clinton said discussions were underway with allies about a formal response, adding that any resumption of talks “will have to be a sustained effort that can produce results.”

The possible diplomatic thaw came as European nations moved to further tighten the economic pressure on Iran. Earlier on Friday, the international financial messaging system known as SWIFT began positioning itself to block key Iranian banks from using the network, which facilitates currency transfers between countries. The European Union has begun drafting a measure requiring SWIFT to exclude certain Iranian banks, effectively depriving Iran of an important means for collecting money from its oil customers.

Iran’s currency has been battered in recent weeks due to the combined pressure of U.S. and European sanctions as well as a threatened European oil embargo scheduled to begin during the summer. U.S. and E.U. leaders say the measures are intended to force Iran to accept strict limits on its nuclear program, which Western governments believe is aimed at giving Iran’s leaders the capability of making nuclear weapons. Iran insists that its nuclear activities are peaceful.

The Iranian letter was itself a long-awaited response to a proposal by Ashton in October on behalf of six world powers. The so-called “P5+1” negotiating bloc — the United States, Britain, France, Russia, China and Germany — formally requested new negotiations but said Iran must be prepared to talk seriously about its nuclear program, without preconditions. The last round of talks between the world powers and Iran ended in failure in Istanbul in January 2011.

Clinton, in her first comments about the Iranian proposal, said the new missive “appeared to acknowledge and accept” the P5+1 group’s framework for new talks, but she added that further clarifications were needed.

“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.

Ashton said the letter pointed to a “potential possibility that Iran may be ready to start talks.”

The possible opening comes at a time when relations between Iran and the West are near historic lows, with Iran threatening to block shipping lanes and cut off oil sales to European countries in response to Western sanctions. Tensions have been further heightened by a series of lethal attacks on Iranian scientists as well as attempted bombings targeting Israeli diplomats in India and the former Soviet republic of Georgia.

The Obama administration, which is phasing in new sanctions targeting Iran’s central bank, has praised European efforts to squeeze Iran through oil embargoes and restrictions on its access to SWIFT, the Belgium-based financial network formally known as the Society for Worldwide Interbank Financial Telecommunication.

David Cohen, the Treasury Department’s undersecretary for terrorism and financial intelligence, said the curbs on Iranian access to SWIFT would build on earlier U.S. efforts to exclude Iranian banks from international commerce.

“It’s another good turn of the screw,” he said.

Cohen said it was too early to assess Iranian intentions in offering new talks, but he expressed hope that international pressure against Iran was achieving the desired effect. He cited the recent threats by Tehran to cut oil sales to Europe as evidence that sanctions were rattling the country’s senior leadership.

“There’s no question that what we’re doing is having a powerful impact,” Cohen said. “The Iranians are under a significant amount of pressure, and they’re reacting in ways that show distress.”