BEIRUT — Syria’s foreign minister said Monday that an internationally brokered cease-fire is still viable, as rescue workers in Aleppo sifted through the rubble from the heaviest air strikes on rebel-held areas of the northern city in five years. Foreign Minister Walid al-Moallem, speaking to Mayadeen TV from New York, also said the government is prepared to take part in a unity government incorporating elements from the opposition, an offer that has been rejected in the past. Opposition activists say more than 200 civilians have been killed in the past week under a sustained aerial campaign that U.N.