Image copyright AFP Image caption The party congress will be the first since Kim Jong-un took control of the country

North Korea has said its ruling Workers Party will hold its first congress in nearly 40 years next month.

The gathering, which will take place in Pyongyang from 6 May, will be only the seventh in the party's history and the first under leader Kim Jong-un.

It will be closely watched for signs of major policy shifts, movement among senior officials or comment on North Korea's nuclear programme.

It comes as North Korea is believed to be preparing a fifth nuclear test.

North Korea has often timed its controversial tests to coincide with big political occasions.

Its fourth test, in January, was followed by the launch of a satellite.

Both were violations of existing sanctions and resulted in the UN imposing further measures limiting trade and contact with the North.

The last North Korean congress was in October 1980, before the current leader Mr Kim was born.

It lasted four days and among other issues saw Kim Jong-il formally named as the intended successor to then leader Kim Il-sung.

Expectation has been growing for months that the leadership was about to announce the seventh congress.

Image copyright Reuters Image caption The last congress, in 1980, saw the formal naming of Kim Jong-il as successor to Kim Il-sung

The statement from North Korea's KCNA news agency on Wednesday gave no details of the event, and did not specify how long it would last.

But it is widely expected that Mr Kim will use the gathering to both reinforce his role as Supreme Leader and to push his agenda of economic development coupled with nuclear progress.

Image copyright EPA Image caption The North may also be preparing another medium-range missile launch

South Korean President Park Geun-hye said on Tuesday that the North had finished preparations for its fifth nuclear test and could carry it out it any time.

It would be the latest show of defiance to the international community in recent weeks, which have seen a series of missile launches.

The North also claimed last week to have used "cold launch" technology to fire a missile from a submarine, while South Korean officials say it also appears to be preparing another test launch of its medium-range Musudan ballistic missile.