North Korea has launched a military spy satellite, according to the South Korean military.

The South Korean Joint Chiefs of Staff said Tuesday night that the North pushed ahead with the planned launch, a day earlier than the previously announced window between Wednesday and the end of the month.

The JCS said that the military detected a North Korean rocket fired at around 10:43 p.m. from Tongchang-ri, where the Sohae Satellite Launching Station is located.

The projectile flew over the sea between the Korean Peninsula and China.

It is not immediately certain if the launch was successful.

North Korea attempted to launch a military spy satellite in May and in August this year but the two attempts ended in failure.

Earlier on Tuesday, the North informed Japan, the regional coordinator for the International Maritime Organization, of its plan to launch a rocket carrying a military reconnaissance satellite within this month.

South Korea's National Intelligence Service said earlier this month that North Korea would carry out its third launch with Russian assistance.

Russian President Vladimir Putin had hinted that he would help Pyongyang with its satellite program during his meeting with regime leader Kim Jong-un in September.

South Korean defense ministry spokesperson Jeon Ha-kyu told reporters earlier on Tuesday that if the North goes ahead with the launch, South Korea’s military will take necessary measures to guarantee the safety of the people.

South Korea, the United States and Japan believe that North Korea's satellite program is in violation of United Nations Security Council resolutions banning the use of ballistic missile technologies by the regime.