BEIRUT—Rescuers in the northern Syrian city of Aleppo were working Thursday to pull the dead and wounded from the rubble of a hospital supported by Doctors Without Borders after the facility was hit by what opposition activists said were Russian airstrikes. The Syrian Observatory for Human Rights, a U.K.-based opposition monitoring group, said its activists had counted at least 27 dead, including the three doctors. Neither the Observatory nor the international medical aid group said who had carried out the airstrikes, but two antigovernment groups—the Syria Revolution Network and the Local Coordination Committees—blamed Russia. In Moscow, the Russian Foreign Ministry...