There's still space for diplomacy in the attempts to resolve Iran's nuclear standoff with the West, U.S. Secretary of State Hillary Clinton said on Thursday, adding that Iran has to prove that its nuclear program is peaceful.

Speaking in a joint press conference with Libyan Prime Minisnter Abdurrahim ElKeib, Clinton said that, as U.S. President Barack Obama said, "we continue to believe we have space for diplomacy."

"It is obviously coupled with very strong pressure in the form of the toughest sanctions that the international community’s ever imposed," the top U.S. official said, adding that Washington wanted "to begin discussions with Iran."

"They insist that their nuclear program is purely peaceful and if that’s the case, then openness and transparency, not only with the P-5+1 but also with the IAEA and the Security Council and the international community, is essential. That’s why we want to respond as we did, positively, to the letter that came from the Iranians," she said.

Clinton was referring to a letter reportedly sent by Iran to European Union foreign policy chief Catherine Ashton, expressing Iran's willingness to renew nuclear talks that fell through a year ago.

Following the letter's appearance last month, Clinton said the U.S. thought it was "an important step and we welcome the letter."

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."

On Tuesday, Ashton announced that Britain, China, France, Germany, Russia and the United States agreed to restart talks with Iran over its controversial nuclear program, saying: "Time and venue of these talks will now be agreed."