WASHINGTON, Jan. 20 (Xinhua) -- The United States and Germany on Friday challenged Iran to open serious dialogue with the world powers.

U.S. Secretary of State Hillary Clinton told reporters that she and German Foreign Minister Guido Westerwelle discussed "at some length" their shared concerns regarding Iran's nuclear ambitions.

Clinton urged Iran to either come back to the table to address concerns over its nuclear program, or face "increasing pressure and isolation." "I want to underscore: We do not seek conflict," she said.

"We strongly believe the people of Iran deserve a better future." Westerwelle agreed by saying that "The door for serious dialogue remains open, but the option of nuclear weapons in Iran is not acceptable to both of us."

 "And this is not only our raison d'etre to protect Israel. It is also a question of the balance in the region. And it's also unacceptable if we look to the situation and to noNPRoliferation necessity worldwide," he said.

Clinton said they will await Iran's response to the letter sent by EU foreign policy chief Catherine Ashton on Oct. 21 last year, which was made public earlier Friday.

The letter, sent on behalf of Britain, China, France, Russia and the U.S., the five permanent UN Security Council members, plus non- permanent member Germany, states that the overall goal of the so- called P5+1 remains a "comprehensive negotiated long-term solution " which restores international confidence in the peaceful nature of Iran's nuclear program.

The P5+1 last had talks with Iran in Istanbul, Turkey in January 2011, but made no progress.