For some better context using numbers everyone can understand: 5 km to 6 km is ~16,400 feet to ~19,700 feet. Airliners normally fly around 35,000 feet give or take a couple thousand. Military aircraft can fly upwards of at least 50,000 feet.
Only a handful of countries use meters for measuring altitude, and as far as I'm aware they are feeling the pressure to reform to using feet for sake of consistency (read: safety).
Besides, denoting both meters and feet like I did will guarantee everyone will understand one way or another.