VIENNA - On the eve of an international conference aimed at ending Syria's four-year-old civil war, Iran signaled it might be willing to back away from its insistence that President Bashar al-Assad remain in power.Iran could accept a six-month transition period at the end of which Assad's fate would be decided in nationwide elections, a senior official from the Middle East familiar with the Iranian position told Reuters on Thursday.During a war that has killed more than 250,000 people and driven 10 million from their homes, Assad's main ally, Tehran, has been locked out of a succession of international peace conferences, all of which ended in failure.Friday's talks are the first high-level discussions for which Washington's allies dropped their objections to including Iran.On Thursday the United States said it was hoping to hear from Iran and Assad's other main ally, Russia, on whether they would be ready to abandon the Syrian leader, whom Washington and its European and Arab allies blame for years of bloodshed.Tehran suggested it was ready to be flexible."Iran does not insist on keeping Assad in power forever," Iranian Deputy Foreign Minister Amir Abdollahian, a member of Tehran's delegation at the Syria talks on Friday, was quoted by Iranian media as saying.The senior official from the Middle East familiar with the Iranian position said that could go as far as ending support for Assad after a brief transition period."Talks are all about compromises and Iran is ready to make a compromise by accepting Assad remaining for six months," he said. "Of course, it will be up to the Syrian people to decide about the country's fate."US and European officials voiced skepticism, however, about the possibility of a breakthrough at Friday's conference, which was expected to include foreign ministers and senior officials from 17 countries.PRINCIPLESIn Washington, White House spokesman Josh Earnest told reporters it was not clear whether Tehran was ready to use its influence to hasten a political transition in Syria."That continues to be up in the air," he said. "But we'll see. To exclude Iran and Russia from these conversations would be a missed opportunity."German Foreign Minister Frank-Walter Steinmeier said in Athens that Friday's talks would be a success if the participants could agree on some basic principles, such as maintaining Syria's territorial integrity and a process for creating a transitional government."The breakthrough will not come tomorrow," he said.Iranian and Russian officials have repeatedly said the priority for Syria should be the defeat of Islamic State militants, who have seized large areas of Syria and Iraq.The divide between Assad's allies Russia and Iran and Western and Arab nations seeking his ouster has deepened since Moscow began air strikes against opposition forces in Syria a month ago.Russia says it is bombing Islamic State, but most of its air strikes have hit other groups opposed to Assad, including many that are supported by Washington's allies.Russia has called for elections in Syria.The list of participants on Friday includes US Secretary of State John Kerry, Russian Foreign Minister Sergei Lavrov, Iranian Foreign Minister Mohammad Javad Zarif, and their counterparts from Turkey, Saudi Arabia, Qatar, Oman, the United Arab Emirates, Egypt and Europe's biggest powers.The meeting follows a smaller one last week in Vienna, a city through which tens of thousands of refugees fleeing conflicts in Syria, Iraq and Afghanistan have passed since last month in Europe's biggest migration crisis since World War Two.