Syrian opposition struggles for unity as battle rages ISTANBUL (Reuters) - Syria's opposition resumed talks on Saturday aimed at closing their fractious ranks, as government forces launched a fierce onslaught on a rebel-held border town to try to gain the upper hand in the civil war. A failure of the opposition to unite could weaken the hand of Russia and the United States, co-sponsors of a proposed peace conference on the war, which has killed 80,000 and threatens to spill over borders and whip up wider sectarian violence. The U.S. and Russian foreign ministers are to meet in Paris on Monday to discuss how to shepherd Syrian President Bashar al-Assad and the opposition into the talks in Geneva. As opposition leaders met in Istanbul, Assad's forces reinforced by Iranian-backed Shi'ite Lebanese Hezbollah fighters unleashed heavy artillery and tank fire to try to seize more rebel terrain in the Sunni Muslim border town of Qusair on Saturday, sources on both sides said. Syria is becoming a proxy conflict between Shi'ite Iran which backs Assad, whose Alawite faith is an offshoot of Shi'ism, and Arab states such as Saudi Arabia and Qatar which support Assad's mostly Sunni enemies. George Sabra, the acting head of the opposition Syrian National Coalition, said thousands of fighters from Iran and Hezbollah were involved in the attack on Qusair, close to the Lebanese border, and in battles in the capital Damascus. Hezbollah leader Hassan Nasrallah said his group would stay in the Syrian conflict "to the end of the road" and would win the war for Assad's government. "We accept this responsibility and will accept all sacrifices and expected consequences of this position," he said in a televised speech, speaking from an undisclosed location. We will be the ones who bring it victory, God willing. More than 22 people in opposition-held areas were killed by Saturday afternoon, most of them rebels, and dozens wounded, according to pro-opposition monitoring group the Syrian Observatory for Human Rights. The pro-opposition Syrian Network for Human Rights said 73 people were killed by Assad's forces, and opposition campaigner Adib Shishakly said Nasrallah lost 75 fighters in the battle for Qusair and that rebel defenders were doing "an excellent job." The United States, concerned by the rising influence of hardline Islamists, has pressed the Syrian National Coalition to resolve its divisions and bring more liberals into the fold. Sources at the coalition, which began its third day of meetings, said major players would focus on such international demands for a broadening of the Islamist-dominated group, leaving leadership issues for later. In Addis Ababa, on the sidelines of an African Union summit, U.S. Secretary of State John Kerry appealed to U.N. Secretary General Ban Ki-moon "to try to get something moving with respect to Syria," according to a pool reporter. Saudi Arabia, the most powerful Arab adversary of Assad, will want to see the Geneva conference, which could convene in the next few weeks, put the exit of Assad at the top of the agenda, diplomats and coalition members said. The inability of the coalition to alter its Islamist-dominated membership and replace a leadership damaged by power struggles is playing into the hands of Assad who, according to Russia, intends to send representatives to the peace conference. While the opposition remained riven by differences, the assault by Assad's forces and their Hezbollah allies on Qusair over the past week is evolving into a pivotal battle. Additional reporting by Erika Solomon in Beirut and Arshad Mohammed in Addis Ababa; Editing by Pravin Char