Syrian opposition resumes tough talks on unity for peace push ISTANBUL (Reuters) - Syria's opposition resumed talks on Saturday aimed at closing their fractious ranks, crucial to launching an international peace conference, and government forces pressed an onslaught on a rebel-held town to try to gain the upper hand in civil war. Failure of the opposition to unite could weaken the hand of conference co-sponsors Russia and the United States in ending Syria's conflict, which has killed 80,000 people, threatens to spill across borders and whip up wider sectarian conflict. 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 peace talks in Geneva the two world powers have jointly proposed. As opposition leaders met in Istanbul, Assad's forces reinforced by Lebanese Hezbollah fighters unleashed heavy artillery and tank fire to try to seize more rebel terrain in the border town of Qusair on Saturday, sources on both sides said. More than 22 people in opposition-held areas were killed by Saturday afternoon, most of them rebels, and dozens were wounded, according to the Syrian Observatory for Human Rights. Assad's forces are believed to have seized about two-thirds of Qusair and largely surrounded the rebels. But the price was high and rebels insisted they were preventing further advances. An official close to Hezbollah told Reuters: "We are in the second phase of our plan of attack but the advance has been quite slow and difficult. The insurgents see Qusair as a critical battle to preserve cross-border supply lines and deny Assad a victory they fear may give him the edge in the prospective peace talks next month. Sources at the Syrian National Coalition (SNC), which began its third day of meetings, said major players would now focus on international demands for a broadening of the Islamist-dominated group, leaving leadership issues for later. Attempts to strike a grand bargain involving veteran liberal campaigner Michel Kilo and businessman Mustafa al-Sabbagh, Qatar's point man in the coalition, went nowhere in talks that stretched overnight, senior coalition sources said. "We are back to square one," one of them told Reuters. On the sidelines of an African Union summit in Addis Ababa, 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. Ban told Kerry he and his special Syria envoy Lakhdar Brahimi "are working very hard to convene, to make this Geneva conference a success." Concerned by the rising influence of hardline Islamists, the United States has pressed the opposition coalition to resolve its divisions and bring more liberals into the fold. Saudi Arabia, the most powerful Arab adversary of Assad, has agreed to play a more active role in furthering the coalition cause, diplomats and coalition members said. Saudi Arabia, the sources said, 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. But they said Russia, a longtime ally of Assad, wanted it to focus on a ceasefire although there is scant rapport between opposition politicians abroad and rebels inside Syria. The inability of the coalition to alter its Islamist-dominated membership as demanded by international backers and replace a leadership undermined by power struggles is playing into the hands of Assad who, according to Russia, intends to send representatives to the peace conference. "The coalition risks undermining itself to the point that its backers may have to look quickly for an alternative with enough credibility on the ground to go to Geneva," a senior opposition source at the talks said. Senior opposition figures said the coalition was likely to attend the conference, but doubted the meeting would secure their central demand - an immediate deal for Assad to quit. While the opposition remained riven by differences, the assault by Assad's forces and their Shi'ite Muslim Hezbollah allies on Qusair, a Sunni town near Lebanon over the past week, is evolving into a pivotal battle. Qusair controls access to Syria's Mediterranean coast, the heartland of Assad's minority Alawite community, and the battle may prove a weighty test of his ability to withstand the revolt. Assad is backed by Shi'ite Iran and Hezbollah against the mainly Sunni rebels supported by Saudi Arabia and Qatar. Hezbollah's intervention is hardening fears that the civil war will cross borders at the volatile heart of the Middle East. The diplomat was referring to a deepening sectarian divide between Shi'ite and Sunni Muslims in Lebanon, where Syrian troops were present for 29 years, including for most of the Lebanese civil war that ended in 1990. The death toll in the northern Lebanese city of Tripoli reached 25 on Saturday in the seventh straight day of clashes between Alawite and Sunni factions backing opposing sides in Syria's war, security sources said. Additional reporting by Erika Solomon in Beirut and Arshad Mohammed in Addis Ababa; Editing by Mark Heinrich