North Korea on Wednesday formally set May 6 for the opening of a landmark ruling party congress -- the first in nearly four decades and an event many fear will be preceded by a fifth nuclear test.

Anticipation over the congress, last held in 1980, has been mounting since the North signalled its intention to hold the gathering way back in October.

Kim Jong-Un is expected to use the event to cement his position as supreme leader and take credit for pushing his country's nuclear weapons programme to new heights.

No details have been provided of the agenda, but it will be scrutinised for any key policy changes or reshuffles among the elite.

The actual starting date had been a closely guarded secret before Wednesday's announcement by the politburo of the central committee of the ruling Workers' Party of Korea.

In a statement carried by the North's official KCNA news agency, the politburo said the congress -- only the seventh in the party's history -- would open on May 6 but did not specify how long it would last.

The 1980 congress took four days, and South Korea's Unification Ministry said it expected next week's gathering to go on for "four or five days".

- Fifth nuclear test? -

There has been growing speculation that North Korea may carry out a fresh nuclear test just ahead of the event as a display of national pride and strength.

On Tuesday South Korean President Park Geun-Hye said the North was understood to have "completed preparations" for a test, and could press the button at any time.

Such a move would constitute a dramatic act of defiance in the face of tough UN sanctions imposed on Pyongyang after its most recent nuclear test in January.

Some analysts have suggested that, by carrying out a fifth test so soon after the fourth, the North might hope to avoid a heavy package of additional sanctions -- but Park insisted that the international community's response would be swift and severe.

"Although the current sanctions are strong, we can impose even stronger sanctions that fill up any holes," the president said.

In recent months the North has claimed a series of major technical breakthroughs in developing what it sees as the ultimate goal of its nuclear weapons programme -- an intercontinental ballistic missile (ICBM) capable of delivering a nuclear warhead to targets across the continental United States.

These claims have included success in miniaturising a nuclear device to fit on a missile, developing a warhead that can withstand atmospheric re-entry and building a solid-fuel missile engine.

Earlier this month Kim monitored the test of an engine specifically designed for an ICBM that he said would "guarantee" an eventual strike on the US mainland.

- Blaming the US -

In a statement carried by KCNA late Tuesday, a North Korean foreign ministry spokesman said US hostility had pushed North Korea into making "drastic progress in bolstering nuclear attack capabilities".

In the run-up to the congress, the North has also carried out a series of provocative missile launches, including the failed test on April 15 of a powerful new medium-range ballistic missile known as a Musudan.

The South's Yonhap news agency on Tuesday quoted unidentified government sources as saying the North had put another Musudan on standby for imminent testing.

Existing UN resolutions forbid North Korea from the use of any ballistic missile-related technology.

The Musudan is believed to have a range of anywhere between 2,500 and 4,000 kilometres (1,550 to 2,500 miles). The lower range covers the whole of South Korea and Japan, while the upper range would include US military bases on Guam.

North Korea has stoked nationalist sentiment ahead of the congress and the South Korean military said Wednesday it had detected another possible propaganda exercise, with the construction of a half-size replica of South Korea's presidential Blue House on an artillery range outside Pyongyang.

"The North is apparently preparing to showcase a mock attack on the Blue House using the replica as a target," said an official with the Joint Chiefs of Staff.