DAMASCUS — Syrian forces battled rebels near the airport in war-battered Aleppo, Syria's state media said Friday, in the first official acknowledgment that fighting has reached the doorstep of the strategic site in the country's largest city. The United Nations announced Friday Algerian diplomat Lakhdar Brahimi will replace Kofi Annan as peace envoy to Syria. UN deputy spokesman Eduardo del Buey said the former Algerian foreign minister and longtime UN official would succeed Annan as joint UN-Arab League envoy. The 78-year-old Brahimi has worked in several high-profile positions at the UN, gaining a reputation as a tough, independent negotiator as envoy to Afghanistan, Iraq and Haiti. He helped negotiate the end of Lebanon's civil war. Brahimi is a member of the Elders, a group of former world leaders working for global peace. In Damascus, activists reported heavy shelling and clashes in many areas, including western districts believed to have rebel pockets. Damascus-based activist Moaz Al-Shami described the shelling as “nonstop” and said gunners were firing from the Qasioun mountains overlooking the city. Syrians are pouring across the border to escape the fighting in their battered homeland and diarrheal disease has broken out in rural areas near Damascus, UN aid agencies said on Friday. The fighting in Syria's two main cities showed the intensity of the civil war and the regime's inability to suppress the rebels despite their overwhelming firepower. Rebel footholds in Aleppo have been the target of weeks of Syrian shelling and air attacks as part of wider offensives by President Bashar Al-Assad's regime. Rebels have been driven from some areas, but the report of clashes near the airport suggests the battles could be shifting to new fronts. Syria's official SANA news agency said “armed terrorist groups” — the regime's phrase for rebels — had been pushed out from areas on both sides of the airport, which is located about 15 km southeast of Aleppo's historical center. Aleppo carries major symbolic and strategic value. It's the hub of northern Syria and close to rebel-held territory and critical supply corridors to the Turkish border. Civilians, meanwhile, have been increasingly caught in the crossfire, and many are fleeing to safety in nearby Turkey. The UN refugee agency spokesman Adrian Edwards said more than 170,000 Syrians have been registered in four neighboring countries. Some 3,500 Syrians fleeing the northern areas of Aleppo, Azaz, Idlib and Latakia reached Turkey's Hatay and Kilis provinces between Tuesday and Wednesday, Edwards said. “There has been a further sharp rise in the number of Syrians fleeing to Turkey,” Edwards told a news briefing. “There are now almost 65,000 Syrians in 9 camps in Turkey, though not all yet formally registered. To put this in perspective, about 40 percent arrived in August.” There has been an outbreak of diarrhea among residents in part of the province of Rural Damascus because the water supply has been contaminated by sewage, the World Health Organization (WHO) said. “In one pocket of Rural Damascus there are 103 suspected cases of E. coli. Laboratory testing is still going on,” Richard Brennan, director of WHO's emergency risk management and humanitarian response department, said. “It is due to contamination of the water supply.” “We have heard of other pockets (of diarrhoeal disease) in other areas of Rural Damascus, but have no details,” he said. Sixty-one children under age 10 are among the 103 cases discovered by health workers in a mobile clinic, WHO spokeswoman Fadela Chaib said. “The local authorities have been alerted and are taking action,” she said. — Agencies