BEIRUT — Syrian troops captured a rebel-held area on the edge of Aleppo on Saturday, tightening their siege on opposition-held neighborhoods in the northern city after what residents described as the heaviest air bombardment of the 51/2-year civil war. The U.N. meanwhile said that nearly 2 million people in Aleppo, Syria’s largest city and onetime commercial center, are without running water following the escalation in fighting over the past few days. Government forces captured the rebel-held Palestinian refugee camp of Handarat as airstrikes pounded rebel-held eastern neighborhoods of Aleppo, killing 52 people, including 11 children and six women, according to the Britain-based Syrian Observatory for Human Rights.