North Korea has announced the first parliamentary elections under Kim Jong Un as the country celebrates the birthday of the young leader who seeks to cement his grip on power after executing his uncle.

The presidium of the Supreme People's Assembly (SPA) decided the election would take place on March 9, the North's official KCNA news agency said on Wednesday.

This year's election will be the first since Kim took power after the death of his father Kim Jong Il in late 2011.

The announcement comes a month after Kim Jong Un had his uncle and top official Jang Song Thaek executed on treason charges.

Observers said the execution was aimed at bolstering Kim's power but also showed his grip on power was not as tight as his father's was.

Kim is expected to use the elections to replace ageing legislators with younger ones loyal to him, said analyst Cheong Seong-Chang at the private Sejong Institute in South Korea.

The country usually holds parliamentary elections every five years, but the polls are largely a formality because candidates are believed to be hand-picked by the ruling Workers' Party.

The last parliamentary vote - a highly staged process with only one approved candidate standing for each of the 687 districts - was held in 2009 under the leadership of Kim's father, Kim Jong-Il.

Kim succeeded his father in December 2011, and the March election will be closely watched for any further revelations on the changing power structure in Pyongyang.