The launch site for several of North Korea's nuclear tests appears to be caving in - a condition known as "Tired Mountain Syndrome".

The geological condition occurs when underground nuclear blasts cause the surrounding rock to become weak and permeable.

Five of Pyongyang's six nuclear tests have been carried out under Mount Mantap at the Punggye-ri nuclear test site, in the country's north west.

Since Pyongyang's most recent - and most powerful - nuclear test on September 3rd, there have been three small earthquakes in the immediate area.

The first happened shortly after the blast while two smaller tremors took place in late September and last week.

The 38 North website, which reports on issues involving North Korea, said there could be concern about the phenomenon at Mount Mantap, as the area affected by the latest blast could potentially extend as far as 1.4km from the detonation point.

But it says such results are "not unusual", and probably won't stop the area being used for further tests.

"There is no valid reason to assume that the Punggye-ri test site is unable to contain additional underground nuclear tests," the report said.

Punggye-ri has two more unused tunnel complexes, it said.

Kim Jong Un told party officials this month that the country's atomic weapons were a "treasured sword" to protect it from aggression.

But nuclear missiles are not the only thing North Korea is launching.

Its deputy United Nations ambassador has said it plans to also launch more satellites.

At a UN meeting called the International Co-operation in the Peaceful Uses of Outer Space, Kim In Ryong said the country's five-year plan for 2016-20 includes the development of "practical satellites that can contribute to the economic development and improvement of the people's living".

He accused the US of trying to block its efforts on grounds it violates UN sanctions.

"The US is the country that launched the largest number of satellites and yet it claims that our launch of satellites is a threat to international peace and security," he said.

Some, including in the US, fear North Korea's space programme is a cover for developing weapons.