BEIRUT (AP) - A coalition of Syria-based opposition groups warned Monday that a proposed international peace conference to negotiate an end to the three-year conflict might be the "last chance" for a solution. The statement by the Coalition of Forces for Peaceful Change is the latest call of support for the talks, which the U.S. and Russia are trying to convene in Geneva by the end of the year. But the so-called "internal opposition," which ranges from officials close to the government of President Bashar Assad to intellectuals and parties that have opposed the rule of Assad's Baath party for decades, has little influence over the disparate armed groups fighting the government. "This is the only available framework and might be the last chance to resolve the crisis in Syria," the groups warned in a joint statement. They welcomed the conference as a chance for global and regional powers to be invested in a solution to the three-year conflict. The coalition called for an immediate ceasefire. The statement came as Syria's main Western-backed opposition group in exile, the Syrian National Coalition, said it intended to attend talks, but made its participation conditional on the creation of humanitarian corridors to besieged areas and the government releasing political prisoners. The often-fractious SNC was still meeting in Istanbul, but an official on early Monday released excerpts of a statement that officials said reflected the outcome of a vote among members. But coalition members were still gathered in Istanbul for an unexpected third day of meetings to hammer out a final position. The group has demanded that Assad and his close allies not be part of any future transitional government. The Geneva talks, as they are known, face other obstacles. The most powerful armed rebel groups fighting in Syria to overthrow the government of Bashar Assad aren't party to the talks. The SNC has little more sway over the groups than the internal opposition. Still, U.S. Secretary of State John Kerry said the SNC's initial statement of attendance was encouraging. This is a big step forward and an important one. Meanwhile, in a blow to rebel fighters, troops loyal to Assad consolidated control of a key military base protecting the airport in the northern city of Aleppo, activists said. The Aleppo International Airport, which has been closed due to fighting for almost a year, is one of the Syrian rebels' major objectives. The Brigade 80 base first fell to rebels in February, but the government took it back this week. By Monday, the Syrian state news agency SANA and the British-Based Syrian Observatory for Human Rights said government forces had taken a series of nearby positions. The Observatory said rebels fighting at Brigade 80 have been led by fighters from the Islamic Tawhid Brigade and two al-Qaida-linked groups, the Nusra Front and the Islamic State in Iraq and the Levant. The Observatory receives its information from a network of activists on the ground. And in the capital Damascus, SANA and the Observatory said a mortar shell that slammed into a vehicle killed a man and his four children. The round landed in the residential area of Jaramana, part of a series of salvos that slammed into the neighborhood on Sunday evening. The Observatory said many more were wounded, including the children's mother. It wasn't clear who fired the shells. There are frequent clashes in a nearby town between Syrian forces loyal to the government of President Bashar Assad and rebels trying to overthrow him. SANA reported the same incident but said three children were killed, along with the father. With reporting by Albert Aji in Damascus and Desmond Butler in Istanbul.