North Korea is ready to carry out a fifth nuclear test and could press the button at any time, South Korea's president said Tuesday, amid reports Pyongyang has readied a powerful, new mid-range missile for an imminent flight test.

Concern has been growing for weeks that the North is building up to another nuclear experiment ahead of a rare, ruling party congress to be held early next month.

"We assess that they have completed preparations for a fifth nuclear test and can conduct it whenever they decide to," President Park Geun-Hye said during a meeting with local media.

If North Korea does go ahead, it would constitute a dramatic act of defiance in the face of tough UN sanctions imposed 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.

- Grave 'miscalculation' -

"North Korea's miscalculation is that by ignoring warnings from the international community and continuing to launch provocations, it will not defend its security, but only speed up its own collapse," she added.

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 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, leader Kim Jong-Un monitored the test of an engine specifically designed for an ICBM that he said would "guarantee" an eventual strike on the US mainland.

The South's Yonhap news agency on Tuesday quoted unidentified government sources as saying the North had readied a medium-range Musudan missile for an imminent test launch.

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

The Musudan is believed to have an estimated 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.

The missile has never been successfully flight-tested.

A test firing on April 15 ended in what the Pentagon described as "fiery, catastrophic" failure -- apparently exploding seconds after launch.

According to the Yonhap sources, North Korea had prepared two Musudans for the test, but the second launch was called off after the first failed.

"The remaining missile now appears to be standing by for launch," one of the sources said.