North Korean leader Kim Jong Un observed the test-firing of strategic cruise missiles, state media reported on Monday, as the US and South Korean militaries kicked off major annual drills that the North views as an invasion rehearsal.

The North’s report on missile tests came three days after the leaders of the US, South Korea and Japan held their first stand-alone trilateral summit and agreed to increase their cooperation on their ballistic missile defences to counter North Korea’s evolving nuclear and missile threats.

During an inspection of a navy flotilla on an unspecified date, Kim boarded a patrol ship to review its weapons and preparations for combat, according to the official Korean Central News Agency. It said Kim later watched the ship’s seamen conduct a drill of launching “strategic” cruise missiles, a word implying the weapons were developed to carry nuclear warheads.

A state media photo showed him watching a soaring missile from the patrol ship from another place, not on the vessel. KCNA said the missiles hit designated targets without any errors, demonstrating the ship’s readiness and attack capability.

Kim said he would bolster efforts to build powerful warships and modernise shipboard and underwater weapons systems for the North’s navy. He called for the country’s sailors to build “overwhelming ideological and spiritual strength”, saying that is more important than numerical or technical superiority of weapons, according to KCNA.