His visit on Sunday to the missile production line came ahead of a planned trip to Beijing to attend a military parade along with Chinese President Xi Jinping and Russian President Vladimir Putin.
North Korea is under heavy international sanctions imposed over its nuclear weapons and ballistic missile programs that were developed in violation of UN Security Council resolutions.
Experts and international officials say the sanctions have lost much of their bite amid growing economic, military and political support from Russia and China.
Kim said that the modernised production process would help increase major missile units' combat readiness, according to state media KCNA.
North Korea has sent soldiers, artillery ammunition and missiles to Russia to support Moscow in its war against Ukraine.
The North's foreign ministry also criticised American co-operation with Japan and South Korea, singling out a recent trilateral joint statement that warned of cybersecurity threats from Pyongyang.
The ministry "strongly denounces and rejects" the United States, Japan and South Korea for using cyberspace as a "theatre of geopolitical confrontation and hostile propaganda", a spokesperson said in a statement carried by KCNA.
"The more the US persists in its anachronistic and malicious hostile acts against the DPRK through the intensified collaboration with its satellite countries, the more distrust and hostility will be piled up between the DPRK and the US," the spokesperson added, using the initials of North Korea's official name.
with AP
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