"So you'll strike once, then twice again," says Gaza watchman Jamal Nasman. He shows no panic, though this call - from an Israeli officer - is deadly serious.Advance warning of an air strike on the building he looks after. And then this. The 13-story building on a quiet street contained residential apartments - and Hamas offices. Israel said Hamas used it for intelligence and military operations.Nasman told Reuters the block, which was evacuated, was his home for many years as well as his place of work."He called me. I thought it was someone from the building. I said, 'I'm listening, what do you want, the generator is working. Are you not coming down?' He told me 'no, stay with me, we want to hit the tower.' I said 'how can you? I felt devastated. I've been in the block for 20 years. It was my life." Amid the fiercest escalation in fighting between Israelis and Palestinians since 2014, the video of the call, captured by an onlooker, gives a rare insight into how Gaza's conflicts unfold.At least, sometimes. Israel confirmed it gave prior warning for civilians to leave. But that doesn't always happen when it strikes what it deems a military target."I told him, 'let me get the residents out, just give me a chance.' He said 'I will give you two or three hours, before hitting the block. First you have to listen. I will fire a warning rocket'. I asked 'a rocket from the drone'? He said 'yes'. He told me 'look up at the drone, we can see you and we know everything. I said OK."Israel says it makes every effort to preserve civilian life and accuses Hamas - which rules Gaza - of using civilian areas to mount operations.Residents of densely populated Gaza say Israeli actions are indiscriminate and aim to punish communities, not just militants. They say most strikes come without warning.