BEIRUT (AP) - Rebels backed by captured tanks launched a fresh offensive on a government complex housing a police academy near the northern city of Aleppo on Sunday, prompting the government to respond with airstrikes to try to protect the strategic installation. If rebels capture the complex on the outskirts of Aleppo, it would mark another setback for President Bashar Assad's regime. In recent weeks, the regime has lost control of key infrastructure in the northeast including a hydroelectric dam, a major oil field and two army bases along the road linking Aleppo with the airport to its east. Rebels also have been hitting the heart of Damascus with occasional mortars shells or bombings, posing a stiff challenge to Assad's regime in its seat of power. On Saturday, opposition fighters in the eastern province of Deir el-Zour overran a site known as al-Kibar, which was home to what is believed to have been a partly built nuclear reactor that Israeli warplanes bombed in 2007. A year after the strike, the U.N. nuclear watchdog determined that the destroyed building's size and structure fit specifications of a nuclear reactor. Syria never stated the purpose of the site. After the bombing, the regime carted away all the debris from the destroyed building and equipment from the two standing structures, analysts said, adding that the rebels were unlikely to have found any weapons in the abandoned complex. "It's more or less a shell because the Syrians decided to remove everything inside the buildings," said Mustafa Alani, an analyst with the Gulf Research Center in Geneva. I don't think there's anything left really of any value for the rebels. Rebels have been trying for months to storm the government complex west of Aleppo in the suburb of Khan al-Asal, according to Rami Abdul-Rahman, the director of the Britain-based Syrian Observatory for Human Rights. The facility also includes several smaller army outposts charged with protecting the police academy inside the compound. Aleppo has been the scene of some of the heaviest fighting of Syria's nearly 2-year-old conflict. In July, rebels launched an offensive on the city, Syria's largest and one-time commercial capital, and quickly seized several neighborhoods. The battle has since devolved into a bloody stalemate, with heavy street fighting that has left whole districts in ruins and forced thousands to flee. Rebels have also been trying for weeks to capture Aleppo's International Airport. There were no reports of fighting for the airport on Sunday, but there have been battles around a section of the highway the army has been using to transport troops and supplies to a military base within the airport complex. On Friday, regime forces fired three missiles into a rebel-held area in eastern Aleppo, flattening several buildings and killing 37 people, according to the Observatory. It said the strike apparently involved ground-to-ground missiles. The Observatory reported a similar attack on Sunday on the town of Tal Rifat, some 30 kilometers (20 miles) north of Aleppo. There was no immediate word on casualties. Also Sunday, a French freelance photographer Olivier Voisin, who was wounded on Thursday in Syria and taken to Turkey for treatment, died of his wounds at an Istanbul hospital, the French Foreign Ministry said. Voisin is the second French journalist this year to be killed while reporting on Syria's civil war, which has proven to be one of the most dangerous conflicts for reporters to cover. The United Nations says at least 70,000 people have been killed since Syria's uprising against Assad's authoritarian rule began nearly two years ago. A senior Syrian opposition leader said Sunday that his umbrella group has suspended participation in meetings with its Western backers and their Arab allies because of their indifference over the regime's attacks on the Syrian people in Aleppo and in other cities. "Assad has reached the stage of real genocide amid Arab silence and we renounce that," said George Sabra, vice president of the Syrian National Coalition. He spoke to reporters in Cairo after meeting the Arab League chief Nabil Elaraby. On Friday, a statement posted on the Facebook page of Sabra's opposition group said its leaders would not travel to Washington or Moscow for any talks to protest the international community's "silence over crimes committed by the regime." In Washington, the State Department condemned rocket attacks on Aleppo, saying in a statement late Saturday the strikes are the "latest demonstrations of the Syrian regime's ruthlessness and its lack of compassion for the Syrian people it claims to represent." Associated Press writers Ryan Lucas in Beirut and Lori Hinnant in Paris contributed to this report.