Japan said on Friday it scrambled fighter jets after two Russian IL-38 information-gathering aircraft were spotted flying between the Sea of Japan and the East China Sea where Russia is holding a joint naval patrol and exercise with China.

Russian and Chinese navy ships have been jointly patrolling the Pacific Ocean and holding naval exercises in the East China Sea, the Russian Defense Ministry said in a statement on Friday.

"In the East China Sea, an exercise was conducted to replenish ships with water and fuel supplies from support vessels," the ministry said.

"A detachment of ships of the Russian Navy and the PLA Navy is currently operating in the waters of the East China Sea and has covered more than 6,400 nautical miles since the beginning of the patrol."

Russia and China also conducted anti-submarine exercises, repelled a simulated enemy air raid, conducted rescue training at sea and practiced helicopter takeoffs and landings on the decks of warships, the ministry added.

The Russian air force activity came a day after Japan spotted Russian and Chinese naval ships crossing waters between the southern Japanese islands of Okinawa and Miyako.