RAFAH, Gaza Strip — Hamas has begun work on a buffer zone along Gaza’s border with Egypt, the Islamic militant group said Wednesday, as part of an effort to assure Cairo that it’s serious about preventing the cross-border flow of weapons and militants. Hamas officials hope the creation of the buffer will lead to an easing of the crippling decade-long blockade of the coastal territory. Earlier this month, Egypt invited a high-ranking Hamas delegation for rare negotiations in Cairo.