It all started when Pyongyang claimed to have successfully launched a spy satellite into space on Tuesday.

This led to South Korea part suspending the agreement, saying it would resume surveillance flights along the border.

Pyongyang is now vowing to fully suspend the pact and send stronger forces and equipment to the border.

"From now on, our army will never be bound by the September 19 North-South Military Agreement," it said in a statement.

It promised to withdraw all measures "taken to prevent military conflict in all spheres including ground, sea and air", and deploy "more powerful armed forces and new-type military hardware" in the border region.

Pyongyang fired a rocket believed to contain its Malligyong-1 spy satellite late on Tuesday and hailed the launch as a "success".

South Korea's military later confirmed that the satellite had entered orbit but said it was too soon to tell if it was actually functioning.

Seoul strongly condemned the launch - and on Wednesday morning top officials agreed to immediately restart surveillance operations along the border, which would allow the South to monitor North Korean outposts and long-range artillery.

That is a breach of a no-fly zone established under the Comprehensive Military Agreement in 2018 - signed by both nations' leaders in an attempt to de-escalate tensions between their two countries and prevent a conflict erupting.

A US official said the South Korean decision to suspend part of the agreement was a "prudent" response, citing the North's "failure to adhere" to the pact.

The following morning, North Korea said it would restore all conflict-avoiding measures it had halted under the deal, and said Seoul would face the blame if a clash broke out.

Its statement came hours after it fired a ballistic missile into the sea east of the Korean peninsula - its first known weapons firing in more than two months. The South Korean military said the missile was likely to have failed.

Pyongyang insists the launch of the spy satellite is part of its "right to self-defence" - but it has been strongly condemned by South Korea, the US and Japan.

Developing a functioning spy satellite is a major part of North Korea's five-year military plan, set out by its leader Kim Jong Un in January 2021.

The technology could in theory enable Pyongyang to monitor the movement of US and South Korean troops and weapons on the Korean Peninsula, allowing it to spot incoming threats. It would also allow the North to plot its nuclear attacks with more precision.

North Korean state media has claimed the country's leader Kim Jong Un is already reviewing images of US military bases in Guam sent by the new satellite. The BBC has not verified this.

By Phelan Chatterjee
BBC News