North Korea has attempted to fire a ballistic missile off its east coast but the launch appears to have failed, South Korean military officials say.
It is unclear what type of missile it was, but it follows three failed tests of the medium-range "Musudan" missile in April.
The North is banned by the UN from using any ballistic missile technology.
Tensions have run high in the region after Pyongyang's fourth nuclear test in January and numerous missile tests.
Also on Tuesday, the newly promoted vice-chairman of North Korea's ruling Workers Party, Ri Su-yong, arrived in China for what South Korean media said was an unannounced official visit.
China is North Korea's closest ally and trading partner, but ties have been strained by the nuclear test and weapons activity.
It is the first visit since the Worker's Party congress earlier this month in North Korea, the first in 36 years.
Mr Ri, who was travelling with a large entourage, would likely be briefing China on other major developments at the congress, in an attempt to rebuild the relationship, said Yonhap.
South Korea's Joint Chiefs of Staff said the attempted launch happened at 05:20 local time (20:50 GMT Monday) off the North's east coast, near the city of Wonson.
A report in the Yonhap news agency quoted sources saying this fourth failed test may also have been of a Musudan missile.
A Musudan missile, in theory, has the range to reach Japan and the US territory of Guam, however the North has never successfully tested one.
During the party congress, North Korea's leader Kim Jong-un extended an offer of military dialogue to the South. But Seoul dismissed the proposal as insincere.
At the same meeting, Mr Kim also vowed to continue with the nuclear weapons programme.
Pyongyang has claimed a series of technical breakthroughs in recent months, including miniaturising a nuclear warhead to fit on a missile, that experts have treated with scepticism.