By Linda Sieg and John Ruwitch TOKYO/SHANGHAI, Feb 4 (Reuters) - Two Japanese F-15s scramble as a Chinese plane nears the disputed islands: one in the lead, the other providing cover. They issue radio warnings to leave the area, but are ignored. Visual wing-tipping signals go unheeded. The Japanese pilots consider their last option: firing warning shots - a step Beijing could consider an act of war.