Is internship a requirement before applying to residency in the US?

It is not a requirement to have completed a foreign internship to join a residency in the US if this what you mean by your question.

Medical residencies in the US are of 2 types in general depending on the specialty:

1- Either it includes internship integrated with the program so you match into PGY1 and continue residency in the same program

2- Or the program start with PGY2 but has a requirement of doing a US PGY1 hence you have to apply to both positions through the match in the same season where you will match to 2 different programs PGY1 and PGY2 simultaneously one staring same year and the PGY2 starting the next year.

If you match to a PGY1 only you can complete that year as hands on 1 year paid USCE and apply the next year to PGY2 positions. If you match to PGY2 only then you have either to find a PGY1 outside the match or forfeit your position and apply again from scratch the next season.