Update 5.46pm: South Korea's joint chiefs of staff said that a missile was launched from North Korea's northern Jagang province today.
There was no immediate confirmation of the launch by North Korea. The day's broadcast on state-run television had already ended when the news broke at around midnight Pyongyang time.
Yoji Koda, a retired admiral in Japan's maritime self defence force, said information that the missile flew 45 minutes and landed west of Hokkaido suggests that it was most likely another ICBM.
Japan's prime minister said North Korea has fired what is believed to have been a missile which may have landed in the sea off the Japanese coast.
Shinzo Abe said officials are analysing the apparent launch.
He has also called a meeting of the national security council.
In Washington, Pentagon spokesman, Navy Captain Jeff Davis, confirmed that the launch of a ballistic missile from North Korea had been detected.
He said: "We are assessing and will have more information soon."
Japanese government spokesman Yoshihide Suga said the missile flew for about 45 minutes and landed off the coast in waters between Japan and the Korean Peninsula.
Public broadcaster NHK said the coast guard has issued safety warnings to aircraft and ships.
July 27 is a major national holiday in North Korea, called Victory in the Fatherland Liberation War Day.
North Korea generally waits hours or sometimes a day or more before announcing launches, often with a raft of photos in the ruling party newspaper or on television news.
North Korean leader Kim Jong Un is usually shown at the site to observe and supervise major launches.
Late-night launches are rare. North Korea usually conducts its missile and underground nuclear tests in the morning.
To have a real deterrent, it is important for North Korea to prove it can launch whenever and wherever it chooses - making it harder for foreign military observers trying to detect their activities ahead of time.
It is likely the North launched the missile at night and from the remote province of Jagang to demonstrate its operational versatility.
Analysts said the Hwasong 14 ICBM launched by North Korea on July 4 could be capable of reaching most of Alaska or possibly Hawaii if fired in an attacking trajectory.
It was launched at a very steep angle, a technique called lofting, and reached a height of more than 1,550 miles before splashing down in the Pacific Ocean 580 miles away.