PYONGYANG (Reuters) - North Korea’s rain-soaked capital was festooned on Tuesday with banners celebrating leader Kim Jong Un ahead of a ruling party congress, as rival South Korea expressed concern that Pyongyang could conduct a nuclear test before or during the rare event.

People pass in front of the building decorated with slogan "The great comrades Kim Il Sung and Kim Jong Il will be with us forever" and their pictures in central Pyongyang, North Korea May 3, 2016. REUTERS/Damir Sagolj

Flower pots lined balconies along streets that have been tidied as part of a 70-day campaign for the first Workers’ Party congress in 36 years, which starts on Friday.

At the congress, Kim is expected to declare isolated North Korea a nuclear weapons state and formally adopt his “Byongjin” policy to push simultaneously for economic development and nuclear capability.

It follows 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.

“Let’s uphold Great Comrade Kim Jong Un’s Songun revolutionary leadership with patriotism!,” one banner read.

Isolated North Korea has conducted a series of weapons tests, including three failed launches of an intermediate-range missile, in the run-up to the Workers’ Party congress.

One banner in Pyongyang extolled a February rocket launch that put a satellite in space. Overseas, however, the launch drew condemnation as a ballistic missile test in disguise.

Kim has aggressively pursued nuclear weapons and could be looking to a successful fifth test this week as a crowning achievement, foreign analysts have said. South Korean Defence Minister Han Min-koo said Pyongyang’s nuclear test may come before or around the time of the opening of the congress.

“North Korea’s goal is to be internationally recognized as a nuclear weapons state,” Han told a parliamentary hearing on Tuesday. “We believe its nuclear capability is advancing.”

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North Korea has invited foreign media to cover the congress, although journalists’ movements are closely managed and much of the country and its people remain off-limits to outsiders.

Pyongyang citizens “fervently welcomed participants of the congress who have given all their patriotic passion ... as a new generation of true warriors of Juche revolution under the leadership of dear comrade Kim Jong Un,” North Korea’s official Rodong Sinmun newspaper said on Tuesday.

Security has been stepped up ahead of the congress.

The Daily NK, a website run by defectors with sources in North Korea, said that since mid-April, free movement in and out of the capital had been stopped and security personnel summoned from the provinces to step up domestic surveillance.

FIRST SINCE 1980

The party congress is the first since 1980, before the 33-year-old Kim was born. His father and predecessor, Kim Jong Il, who died in December 2011, never held one.

While some past party congresses featured representatives from countries the North has ties with, South Korean officials have said they were not aware of invitations sent to official foreign guests for the upcoming event.

North Korea has become increasingly isolated over its pursuit of nuclear weapons and ballistic missiles, and was hit with tightened U.N. Security Council sanctions in March that were backed by its chief ally, China, in response to a January nuclear test.

Pyongyang has conducted a flurry of missile and other weapons tests in the run-up to the congress, although not all have been successful. It made three attempts last month of what was believed to be its intermediate-range Musudan missile, all of which failed, according to U.S. and South Korean officials.

The congress is expected to last four or five days, South Korean government officials and experts said. Kim may decide to take on the post of party General Secretary, a position held by his late father, elevating himself from First Secretary.

“It is now his era, and the elders have passed away, and the idea will be that if he remains first secretary, then he might think he won’t get enough respect because of that,” said An Chan-il, former North Korean military official who now heads a think tank in Seoul.