AMMAN - At least 20 people were killed on Monday in air raids and rocket barrages on opposition strongholds in Damascus to halt a flow of rebel fighters into the capital, activists and opposition military sources said.The bombardment, which concentrated on the north and east of Damascus and on the nearby suburb of Douma, a rebel stronghold, is among the heaviest in the capital since the uprising against President Bashar Assad began two years ago, they said.Ten rebels also were killed in fighting in Adra, a contested residential and industrial town between Douma and Damascus where several army and intelligence compounds are located, and in the suburb of Harasta on the main highway north.