The stopper – who turned 20 on Saturday – has flown out to Mexico, where Team USA are to compete in the 2013 CONCACAF Under-20 Championship over the next few weeks.
Cropper has appeared for his country at under-20 level nine times before, having made his debut in 2010.
Cropper's team play their first group stage fixture this evening as they take on Haiti at the Estadio Olímpico de C.U. in Puebla at 5.30pm local time.
USA then face Costa Rica at the same venue on Friday at 5.30pm, with the winners and runners-up from each group progressing to a quarter-final to be played in the middle of next week.
The four semi-finalists will qualify for this summer's U20 World Cup finals, as well as contesting to win the CONCACAF Under-20 Championship itself.
The semi-finals take place on Friday 1st March, with the final and third-place play-off set for two days later.
Cropper missed out as Saints' Under-21s faced West Ham United last Friday night having travelled out to Mexico earlier in the week, and will remain away for the Development Squad's next two Barclays U21 Premier League fixtures.
If the USA do qualify for the World Cup finals, which take place in Turkey in June and July of this year, Cropper could find himself up against a couple of his Southampton teammates.
James Ward-Prowse, Jack Stephens and Calum Chambers could be named in the England Under-20 squad having appeared at under-19 level in recent months.
England have already qualified for the tournament by winning their group at the 2012 UEFA European Under-19 Football Championship.