North Korea has fired five short-range missiles in a test fire off the east coast.
In preparation for the exercise, ships were banned from the area from 10-20 October.
An official has spoken on condition of anonymity, confirming the report of the launches and saying that parts of the east and west coasts are 'no sail zones'.
Sources have said that the launches appear to be part of regular military exercises, but did not exclude the possibility that Pyongyang is demonstrating its firepower for political purposes.
These launches are the first exercises reported since early July, and come amid international efforts to bring the hardline communist state back to six-nation nuclear disarmament talks.
US Secretary of State Hillary Clinton said: 'The United States and its allies were trying to demonstrate to North Korea that the international community would not accept its continuing nuclear programme.
Hillary Clinton, speaking in Belfast, said: 'Our goals remain the same. We intend to work toward a nuclear free Korean peninsula.'
On 2 July North Korea fired four short-range missiles believed to be surface-to-ship types.
North Korea was criticised by the UN when seven longer-range ballistic missiles were fired on 4 July.