US Secretary of State Hillary Clinton on Tuesday renewed her call to Iran to start serious talks on its nuclear programme, stressing that a window for negotiations will not be open indefinitely. "Our message to Iran is clear: our window remains open to resolve the international community's concerns about your nuclear programme diplomatically ... but that window can not remain open indefinitely," she told reporters in Sarajevo, the first leg of her five-nation Balkans tour. Clinton voiced hope that Tehran would soon start "serious, good faith negotiations" over the issue. Several rounds of negotiations between world powers and Tehran have failed to produce much progress over transparency in an Iranian nuclear programme that the West suspects is a front for developing nuclear weapons. Tehran denies the charge and insists on world recognition of its right to enrich uranium -- something that is excluded by four rounds of UN sanction over its refusal to cooperate with nuclear agency inspectors. The last round of direct talks in Moscow in June included mentions of a new meeting possibly being held in a neutral venue such as the Kazakh capital Astana or Beijing. Meanwhile, Iran and the United States have recently both denied any deal had been reached for one-on-one nuclear talks, as The New York Times reported -- even though the White House said it was open to such dialogue.