BEIRUT (AP) - A dual picture of Syria's rebellion is emerging: Fighters on the ground make advances, seizing territory in the south and even firing one of the heaviest mortar volleys yet into the heart of Damascus on Monday. But at the same time, the would-be opposition leadership is falling deeper into disarray. The dichotomy underlines the difficulties as the U.S. and its allies try to shape the course of the fight to oust President Bashar Assad - and, more importantly, avert chaos in the event the regime is toppled. As the Syrian civil war enters its third year, hopes that the perpetually fragmented opposition would coalesce to form a real leadership for the fighters on the ground seem more elusive than ever. Instead, divisions broke out this week in the main opposition group, the Syrian National Coalition. Its head announced he was stepping down, complaining of restrictions on his work. Amid infighting, 10 other members said they were suspending their membership. The resignation by Mouaz al-Khatib, a respected Muslim preacher seen as a uniting figure and a moderate against the rising influence of Islamic extremists among Syria's rebels, came only days after the SNC narrowly elected a little-known information technology professional from Texas to head a planned interim government as its prime minister. In another blow, the head of the SNC's military branch, Gen. Salim Idris, said his group refused to recognize the new prime minister, Ghassan Hitto, because he lacked broad support among the opposition. Hitto was backed by the Muslim Brotherhood and the Gulf nation of Qatar; many prominent opposition figures boycotted the vote that installed him. Amid the disarray, the Coalition, largely comprised of exiles, has made little mark among the hundreds of independent rebel brigades that are doing the fighting against Assad's forces. Most rebel groups still cobble together their own funding and arms and give little more than lip-service to the authority of Idris' Office of the Chiefs of Staff. Still, rebels have recently been running up successes on the ground. Fighters have been steadily gaining more ground near Syria's southern border with Jordan and Israel. In the north, they have been expanding the territory they hold, recently capturing the city of Raqqa, a series of military bases and the country's largest dam. Rebels have also seized footholds on the edge of the heavily guarded capital and, while they have been unable to break into the city, they have used their positions for mortar barrages, trying to shake the government's grip. On Monday, they fired off a volley of mortar shells that crashed near a landmark downtown traffic circle in the capital, killing two people and wounding several others, state TV said. It was some of the worst shelling in the heart of the city since the rebellion against Assad began in March 2011. "We have gotten used to the sounds, but it saddens me to see the streets of Damascus empty after 6 p.m.," said Youssef al-Ashhab, a 47-year-old civil servant. The mortar barrage struck Damascus' Umayyad Square, at the center of a large intersection west of downtown near the government TV headquarters and less than a kilometer (mile) from Assad's formal residence. It is also near the famous Opera House, often frequented by Assad and his British-born wife, Asma, before the uprising, and the Sheraton hotel, used by U.N. workers in Syria. U.N. spokesman Martin Nesirky said Monday that the U.N. is temporarily relocating some Damascus-based staff of the office of joint U.N.-Arab League envoy Lakhdar Brahimi to Beirut and to the office's main office in Cairo following mortar fire that damaged the hotel and a U.N. vehicle. He said all national staff from Brahimi's office have been asked to work from home until further notice. The international community has been at a loss for ways to stop the bloodshed in Syria, complicated by the lack of cohesive opposition leadership. Hitto's election as interim prime minister of rebel-held territory is unlikely to create greater unity on the battlefield. On Sunday, a rebel military leader was wounded in the foot by a bomb planted in his car in eastern Syria, according to activists and rebels. Col. Riad al-Asaad, a former colonel in the Syrian air force who defected and fled to Turkey in 2011, was the leader of a now-sidelined rebel umbrella group known as the Free Syrian Army. Al-Asaad was said to be hospitalized in Turkey, and there were conflicting reports on whether his foot was amputated after the attack. Al-Asaad was among the first to call openly for armed insurrection against Assad. Initially, most Syrian activists were inspired by the uprisings that had successfully toppled dictators in Tunisia and Egypt and thought popular protests would bring about the same result in Syria. Today, hundreds of independent rebel groups are fighting a civil war against Assad's forces across the country, and many activists no longer bother to stage unarmed protests. Also Monday, the Syrian National Coalition said a delegation was heading to Doha, where the Gulf state of Qatar will host a two-day Arab League summit starting Tuesday. Al-Khatib will head the delegation despite his resignation this week as head of the SNC. He said in a post on his Facebook page that he would address the summit "in the name of the Syrian people." Associated Press writer Albert Aji contributed reporting from Damascus.