(Bloomberg) -- Leader Kim Jong Un said deploying spy satellites is essential to protect North Korea’s sovereignty after one of its rockets failed in flight, as his regime made good on a threat to drop “filth” on South Korea from balloons it sent over the border.

In a speech to the country’s Academy of Defense Sciences, Kim said the rocket launched Monday that was meant to deploy a spy satellite failed due to an abnormality in a first-stage engine. He also paid tribute to the engineers and technicians “who have become a solid cornerstone of the fighting capability of our army,” the official Korean Central News Agency quoted him as saying.

“Possession of a military reconnaissance satellite is a prerequisite and essential for our nation to further strengthen its self-defense deterrence and protect national sovereignty,” KCNA on Wednesday quoted Kim as saying.

The North Korean rocket carrying a spy satellite blew up shortly after liftoff in a setback for Kim, who went to Russia last year seeking help placing an array of reconnaissance probes into orbit.

Kim intends to launch three spy satellites in 2024, KCNA reported after a policy-setting meeting of top officials in the last days of December. The satellites can help him keep an eye on US troops in the region and improve his ability to strike targets as he upgrades his nuclear weapons arsenal.

Read more: North Korea Rocket Explodes in Blow to Kim’s Satellite Plans

In the hours between North Korea giving notice of its intention to launch and actually firing off the rocket Monday, South Korea put on a show of force by sending about 20 warplanes into skies near the border with its neighbor.

Kim said South Korea was playing with fire and the move cannot be ignored. He also reiterated that his state will do all it can to boost its war deterrence against US imperialists and “their stooges” in Seoul.

This was North Korea’s fourth attempt to launch a spy satellite since last year, with three ending in failure soon after lift-off and a launch in November successfully placing a probe into orbit. It remains to be seen if there was any influence from Russia on the design of the new engine, which was rolled out for use after President Vladimir Putin pledged at a September summit with Kim to help him with his space program.

The US and its partners have also accused Kim of sending massive amounts of munitions to Putin to help in his war on Ukraine in return for aid that is advancing North Korea’s military and economy. Moscow and Pyongyang have denied the claims.

South Korea’s military separately said it has detected more than 150 balloons sent from North Korea over the border since Tuesday night that contain trash and other waste, Yonhap News reported. The contents include batteries, shoe parts and manure, Yonhap cited a military official it did not name as saying.

This comes after North Korea’s official media over the weekend cited a vice defense minister as saying the state would scatter “mounds of wastepaper and filth” on the South Korean side for what Kim’s regime saw as security threats that included surveillance flights. 

The move also appears to be in response to South Korean activists earlier this month sending balloons into North Korea that contained anti-Pyongyang messaging. Millions of leaflets sent by South Korean activists and defectors from North Korea have flown across the border for more than a decade bearing messages critical of North Korea’s leaders, fueling friction between the rivals.

--With assistance from Shinhye Kang.

(Updates with balloons from North Korea after threat to send ‘filth’ in final paragraphs.)

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