I want to produce a list with oder by total degree, of monomials in several variables

i.e. {1, x, y, x^2, y^2, xy} would be the list or all monomials in the variables x and y with total degree <=2,

currently i can produce a list using a loop command that gives me the set

U:=(seq(seq((x^n)*(y^m), n=0..2),m=0..2));

which returns the list

U=1, x, x^2, y, xy, x^2y, y^2, xy^2, x^2y^2

which has terms that i do not require i.e. ...