From Business Insider:

A Friday press release out of China confirms the incident began when Beijing was flying a Shaanxi Y-8 on a “routine Thursday patrol” over the “oil and gas fields in the East China Sea.”

…on Thursday Japan spotted aircraft in its Air Defense Identification Zone (above the islands) that it believed to be Chinese J-7 interceptors, along with some J-10 fighters whose combat abilities rival that of Western jets. Japan responded with two F-15s scrambled from Naha, Okinawa — just a couple hundred miles away. There are minor variations from either side about who sent what first, but all agree the aircraft met above the islands.