Geneva 2: Ambitious aim, dim prospects for Syria talks A conference on the long, bloody conflict is due to start in Montreux on Wednesday Negotiators from the Syrian government and opposition won't meet until Friday The different sides appear deeply opposed and hopes of much progress are slim The United Nations invited and then disinvited Iran, adding to tensions (CNN) -- An international conference with the ambitious aim of ending the Syrian civil war is set to begin Wednesday in Switzerland as controversy rumbles on about Iran's association with the talks. Top diplomats from the United States, Russia and other world powers are gathering near the shores of Lake Geneva to discuss how to address a conflict that has killed more than 100,000 people and shows no signs of ending anytime soon. Representatives of the regime of Syrian President Bashar al-Assad and the opposition will also attend the conference, setting around the negotiating table bitter foes who have been engaged in a complex, bloody fight since 2011. The war has become increasingly sectarian, drawing in Syria's regional neighbors and forcing out more than 2 million refugees, many of them children. Analysts say the prospects of reaching any solution to the conflict at the talks are dim, but there are hopes that progress could be made on improving the situation for the most vulnerable victims of the conflict. A preliminary international session is scheduled for Wednesday in Montreux, at the eastern end of Lake Geneva, but the talks between the Syrian government and opposition delegations aren't slated to begin until Friday in Geneva, at the lake's southwestern tip. Iran dispute In an indication of the sobering challenges facing negotiators, squabbles flared before the conference even started. The main Syrian opposition movement, the Syrian National Coalition, took until Saturday to agree to attend in a vote that exposed deep rifts within its ranks. One of the groups under the coalition umbrella blasted it for assenting to participate in the talks, accusing it of heading to Geneva with "a folder of concessions and withdrawals." And furor has surrounded the embarrassing public announcements by the U.N. Secretary-General Ban Ki-moon inviting and then disinviting Iran, a key supporter of al-Assad's regime, to the conference. The controversy over Iran, which is believed to provide military and intelligence support to Syrian government forces, threatened at one point to derail the talks. Russian Foreign Minister Sergey Lavrov chided the United Nations on Tuesday for its abrupt U-turn on Iran, following pressure from the main Syrian opposition group and the United States. Iran had already announced that it wouldn't be attending the peace conference because it would not tolerate any preconditions for joining the talks -- including acceptance of the framework laid out in a previous conference that foresees a transitional government. Lavrov called Ban's reversal a mistake, but "not a catastrophe," adding that Russia and others will push for balanced talks between those representing al-Assad and the rebels. Russian Prime Minister Dmitry Medvedev told CNN's Christiane Amanpour that the talks cannot be taken seriously without Iran. "The thing that has happened with the withdrawal of the invitation to Iran, I believe that's unacceptable," Medvedev said in an exclusive interview to air Wednesday on CNN's Amanpour. Can someone think the Syrian problem can be seriously discussed without the Iranian factor and their account of it? Gruesome Syria photos may prove torture by Assad regime U.S. and Russia meet Lavrov and Kerry met behind closed doors Tuesday at the Fairmont Le Montreux Palace. That meeting was followed by another that included Kerry, Lavrov, Ban and the U.N. special envoy to Syria, Lakhdar Brahimi. They all agreed on the significance of having representatives of the regime and the rebels at the negotiating table, according to a senior State Department official, who spoke on the condition of anonymity. The four also agreed on the importance of "beginning the long process of negotiating a transitional governing body by mutual consent," said the official, who was not authorized to speak publicly on the issue. Kerry also briefly joined a meeting between senior U.S. officials and representatives of the Syrian opposition coalition, according to the official. The peace talks also were front and center in a telephone call between U.S. President Barack Obama and Russian President Vladimir Putin on Tuesday. The two discussed preparations for the talks, according to a statement released by the White House. Besides the United States and Russia, the other permanent members of the U.N. Security Council -- Britain, France and China -- are attending the conference along with more than 25 other countries. Representatives of the Arab League, the Organization of Islamic Cooperation and the European Union are coming, as well. Iran out of peace conference on Syria Bitter opponents The aim of the talks in Switzerland is to set up a transitional government to help end the violence that has wracked the country. The first round of peace talks -- known as the Geneva I communiqué -- calls for a transitional government and eventual free elections as part of a political settlement to end the war. But the different sides in the conflict appear to be deeply opposed. Al-Assad, whose forces have regained momentum against a now-fractured opposition, has said he's not looking at the talks as a way to transition out of power. Al-Assad has called for the conference to include a focus on "fighting terrorism," his government's term for rebel forces. In a meeting with Syria's delegates to the talks, al-Assad directed them to preserve their nation's sovereignty by "preventing and rejecting any foreign interference no matter its form and context," Syria's state-run news agency, SANA, reported Monday. Most outside support for rebel forces has come from the Gulf monarchies of Saudi Arabia and Qatar. CNN's Ben Brumfield, Matt Smith, Chelsea J. Carter, Shirzad Bozorgmehr, Christine Theodorou, Saad Abedine, Samira Said and Nick Paton Walsh contributed to this report.