The Israeli army said that 21 soldiers have been killed in the central Gaza Strip, marking the deadliest single incident for the troops in the more than three-month-old war against Hamas militants. Army spokesman Daniel Hagari, speaking after a news blackout was lifted on Tuesday morning, said the soldiers were killed on Monday afternoon when two nearby buildings exploded and collapsed. The blasts were likely the result of mines that had been planted by the troops to demolish the structures, Hagari said, while cautioning that it was not clear what triggered the blasts and that this was still u…