North Korea claims it has successfully put a military spy satellite into space, after two earlier attempts failed this year.
It comes after a meeting between Russian President Vladimir Putin and Kim Jong Un in September, where Moscow offered Pyongyang help with its space programme.
South Korea says it has not confirmed yet if the satellite is operational.
But it says it believes the North received help from Russia.
Following the launch, South Korea announced it would resume surveillance along its border with the North, thereby taking steps to suspend parts of a deal agreed by the two countries in 2018 aimed at lowering military tensions.
North Korean state news agency KCNA said the satellite, named Malligyong-1, had "accurately" entered orbit and and leader Kim Jong Un had observed the launch.
The launch has been condemned by the UN, which maintains sanctions on North Korea due its nuclear missiles development, and other countries including the US and Japan.
The UN secretary-general said: "Any launch by [North Korea] using ballistic missile technology is contrary to the relevant Security Council resolutions."
He reiterated a call for the North to return to its path of denuclearisation.
The White House called the move a "brazen violation" of UN resolutions, while Japan's Prime Minister, Fumio Kishida, also condemned the launch - which flew over the country's Okinawa prefecture and on towards the Pacific Ocean.
The Japanese government briefly issued an emergency warning, telling residents in Okinawa to take shelter. North Korea had on Monday told Japan it intended to attempt its satellite launch again. It had tried twice this year and failed - in May and August.
Mr Kishida condemned the launch "in the strongest terms" and said a complaint had been lodged with North Korea.
"Even if the purpose is to launch a satellite, using ballistic missile technology is a clear violation of the relevant United Nations Security Council resolutions," said Mr Kishida.
"It is also a very serious matter that greatly concerns the safety of our people."
The launch came earlier than anticipated, outside the nine-day window Pyongyang had notified Japan about. This was supposed to start on Wednesday and end at 23:59 local time (14:59 GMT) on 30 November.
KCNA, citing the North's space agency, said the Sohae launch facility in the northwest of the country, close to the border with China, was used for the operation.
North Korea's claim that it has successfully launched the spy satellite has not yet been verified.
If the rocket is confirmed to be a spy satellite, it would be the third attempt by the North to launch one this year.
Earlier, Japan said it would work with South Korea and the US to "strongly urge" the North not to proceed with the launch, which they said would violate UN resolutions.
The Japan Coast Guard said Pyongyang's notification designated three maritime zones believed to be the areas where debris from the rocket carrying the satellite will fall.
Two are to the west of the Korean Peninsula and the other is to the east of the Philippines' island of Luzon.
Kang Ho-pil, chief director of operations at the South Korean Joint Chiefs of Staff, warned that Seoul would take "necessary measures" should the launch proceed.
A spy satellite is a coveted prize for Kim Jong Un, as it will allow him to monitor incoming attacks and plot his own more accurately.
However the United Nations Security Council has banned Pyongyang from launching satellites because it sees them as a pretext to test the North's missile technology.
South Korea retrieved debris from the North's first launch in May and said the satellite had "no military utility". After a second attempt in August failed, Pyongyang's space agency said it would try again in October but did not do so.
Russian President Vladimir Putin suggested in September after a meeting with Mr Kim that Moscow could help Pyongyang build satellites, but the details of what was actually promised was unclear.
Earlier this month, South Korea announced plans to launch its own spy satellite by the end of November. The satellite is to be carried by a rocket from US company SpaceX.
This is reportedly the first of five spy satellites Seoul plans to launch into space by 2025.