WASHINGTON (AFP) - US Secretary of State Hillary Clinton said Tuesday "the burden remains on Iran" to show it is serious about tackling the international community's concerns over its nuclear ambitions. Clinton's remarks highlighted the continued deadlock after Iran said the ball was in the court of the six major powers that have been working to resolve concerns over Tehran's nuclear program. The United States, Russia, China, France, Britain and Germany have engaged in on-and-off talks with Iran aimed at halting its uranium enrichment program, one which Washington fears masks a drive to produce a nuclear weapon. Iran, which insists its program is peaceful and denies the US charges, said it is ready for new talks after a meeting in Istanbul ended without progress in January, adding the ball is in the court of the six. But Clinton, speaking during a press conference with European Union foreign policy chief Catherine Ashton who represents the six powers in contacts with Iran, effectively fired right back. "Let me make clear that the burden remains on Iran to demonstrate it is prepared to end its stalling tactics, drop its unacceptable preconditions and start addressing the international community's concerns," Clinton said. While the six want to focus on Iran's nuclear program, Tehran wants to raise issues like global security and nuclear disarmament, Israel's suspected atomic arsenal and the right of all countries to civil nuclear cooperation. In a letter to Ashton earlier this month, Iran restated its conditions for a resumption of talks. "It's taken three months for the reply to come. I had wished for a stronger and better letter from them to recognize that the offer on the table is an offer they should look at very carefully," Ashton told reporters. "I will be sending a reply. We'll be consulting with our partners, not least with the United States, before we do so. But I do urge Iran to think again and to consider coming back to the table," she said. Asked if there would be a new round of talks involving the six powers and Iran, Ashton replied: "I would like to say there will be a new round of talks. From the letter that I've received, I don't see that at the present time." The UN Security Council has adopted four sets of sanctions against Iran, the most recent in June last year, over its refusal to suspend uranium enrichment, the sensitive process that lies at the heart of Western concerns. A panel of experts that monitors the sanctions said Iran was circumventing them but that its nuclear work had been impaired.