SEOUL (REUTERS) - North Korean leader Kim Jong Un is poised to declare his signature ruling policy during a rare party congress in May and despite tough new UN sanctions, it is likely to be the twin pursuit of nuclear prowess and economic development.

North Korea's official media have carried almost-daily reports and commentary extolling the "sacred road" to the day Kim's leadership will be endorsed at the "victorious and glorious 7th party congress".

This week marks the half-way point in what state media describes as a 70-day "top-speed dash to glorious victory", a campaign that calls for ramped-up productivity and a sprucing-up of the capital Pyongyang as it prepares to greet delegates.

Propaganda posters in photos seen by Reuters urge residents: "Comrade, have you implemented your 70-day battle plan for today?"

The highlight of the congress, the first in 36 years, will likely be the formal adoption of Kim's signature "byongjin policy", said Mr Cho Min, an expert on the isolated North's leadership and former vice-president of the South Korean government-run Korea Institute for National Unification.

Byongjin means "simultaneous push" - in North Korea's case for economic development and nuclear weapons capability. It follows Mr Kim's father's Songun, or "military first", policy and his grandfather's Juche, the North's home-grown founding ideology that combines Marxism and extreme nationalism.

"He (Kim Jong Un) doesn't look willing to back down on nuclear armament and the congress is the place for maximum impact if he wants to declare it to the world," Mr Cho said, adding another nuclear test is "worryingly" likely in the run-up to the meeting.

North Korea conducted its fourth nuclear test on Jan 6, leading to new United Nations Security Council sanctions early this month, backed by Pyongyang's sole major ally, Beijing.

Since taking power in late 2011 after the death of his father Kim Jong Il, Mr Kim Jong Un's tenure has been marked by turmoil and purges within his inner circle, including the 2013 execution of his powerful uncle Jang Song Thaek as well as five replacements of his defence chief.

"He's tried other things such as (Workers' Party) Political Bureau meetings, dismissals, demotions, etc., and it hasn't had the effect he wanted," said Mr Michael Madden, an expert on the North Korean leadership.

"The party congress is the most effective way to re-set North Korean political culture," he said.

BATTLE PLAN

The North is probably in the midst of selecting more than 3,000 delegates to the event that revises the ruling party's charter and adopts new rules for its ultimate leadership body, officials in South Korea said.

It is a process that will continue until about 20 days before the congress, expected in early May, while the agenda is formalised at the same time, one of the officials said, based on the run-up to the last two meetings, in 1970 and 1980.

"The preparations are no simple feat given they have to arrange to find lodging for 3,000 people, for example," another official said, speaking on condition of anonymity.

Pyongyang residents have been mobilised before daybreak and after work, as well as during regular work hours, to make preparations, a Western source in Pyongyang told Reuters, requesting anonymity. "People are really tired, and some of my contacts fall asleep while meeting me," the source said.

The last party congress, 36 years ago, unveiled a new generation of party apparatchiks and a reshuffling of the ruling apparatus to ready for Mr Kim Jong Il's rule, which only began 14 years later when his father died.

Once he took power, however, Mr Kim Jong Il is believed to have ceased all formal party management. "In many ways, Kim Jong Il ruled the country in open violation of the party charter," Mr Madden said.

Mr Kim Jong Un, believed to be 33, is likely to continue taking steps to restore the party as the centre of administration, and could possibly announce measures to lift restrictions on the thriving informal economy.

Mr Cheong Seong Chang, a senior analyst at the Sejong Institute, said Mr Kim appears "obsessed with nuclear weapons development", which he views as cost-effective compared to expenditure on a conventional military and weapons, allowing him to save resources he can divert to light industries.

"Under Kim Jong Un, things have improved in terms of the economy," Mr Cheong added. "More bicycles are on the road and there's more fish and more greenhouses."