the slightly less safe ICAO phraseology.
Not quite, after the Flying tigers 747 accident in KL in 1989, ICAO recalled that you must always use the words " altitude" or " FL" before enunciating numbers
it is not climb to " it is "climb to FL ( or altitude) xxx
In the US they use " maintain" probably because it is far easier to pronounce than " altitude" as they have a rather high transition altitude .( my guess
)