A Chinese military plane was spotted flying in South Korea's special air defence zone for nearly four hours, raising suspicions that the aircraft was on a surveillance mission. South Korean authorities say the plane had not notified them prior to its arrival.

As many as 10 fighter jets were scrambled by the South Korean air force to closely monitor the movement of the Chinese aircraft. South Korea's Joint Chiefs of Staff (JCS) said the plane flew close to South Korean territorial airspace.

"Our military warned it to stop the act of raising tensions that can trigger an accidental conflict through the South Korea-China (military) hotline and (pilot's) radio communication," the JCS said in its statement.

South Korea said the flight was "unusual" and suspects it could have been deployed to specifically test Seoul's response.

The country's foreign ministry and the military are expected to register formal protests with their Chinese counterparts over the incident.

This is not the first time a Chinese aircraft is entering the air defence zone since Beijing routinely sends its flights in the region as a show of its aerial might.

South Korea, Japan and China have specially designated air defence zones in the region, all of them overlapping one another near Leo Island. All the three nations are also involved in territorial disputes among themselves.