The presidents of Russia and the United States have discussed Iran's nuclear program before a six-nation meeting on ending months of disagreement between Washington and the Kremlin on how to persuade Teheran to stop uranium enrichment.The phone conversation Tuesday between US President George W. Bush and Russia's Vladimir Putin reflected the gravity of what will be discussed at Thursday's talks in Vienna, where six countries hope to sign off on incentives and penalties designed to wean Iran off enrichment.Teheran says it wants the technology to generate energy, but there is growing international concern it could use it to make nuclear weapons. "Both leaders spoke in favor of the further development of international efforts in the interests of resolving the Iranian nuclear problem," the Kremlin said in a statement about the phone call.Bush also spoke by phone to two other key European players in the Iran negotiations, French President Jacques Chirac and German Chancellor Angela Merkel, the White House said, providing no further details about the conversations.In Brussels, EU foreign policy chief Javier Solana said Iran must prove it is not pursuing nuclear weapons. It can do so, he said, by accepting the incentives that the Vienna gathering hopes to finalize as part of a package meant to reward Iran if it gives up enrichment activities - or penalize it if it doesn't, possibly through UN Security Council sanctions."If they reject it, it will be once again a clear sign they are looking ... to enter nuclear weapon-type of enrichment that for us will be very dangerous," Solana told the European Parliament's foreign affairs committee. Solana suggested that if approved in Vienna by the US, Russia, China, France and Britain - the five permanent Security Council members - plus Germany, the package would be formally offered to Teheran before a June 21 EU-US summit in the Austrian capital.Indicating that US-Russian differences remain before Thursday's Vienna meeting, he said: "I hope we still have some room for maneuver."In Malaysia, Iranian Foreign Minister Manouchehr Mottaki said his country was ready for "negotiations on Iran's nuclear issue without any preconditions."Still, Teheran has for months insisted that its right to conduct nuclear enrichment is nonnegotiable - and nothing in Mottaki's comments suggested that stance had changed. If Iran remains defiant, the rewards component of the package up for approval in Vienna would become void and be replaced by punishment to be decided by the Security Council.US State Department spokesman Sean McCormack brushed off the Iranian offer to talk, saying Tuesday in Washington: "We've heard that before."Any package foreign ministers approve Thursday would be presented to Teheran by France, Britain and Germany - the trio that broke off talks with Iran in August after Teheran resumed activities linked to enrichment.The Security Council gave Iran until the end of April to suspend all such activities. Instead of complying, Iran announced last month that it had for the first time successfully enriched uranium and was doing research on advanced centrifuges to produce more of the material in less time.