with a matrix of distance, create several group of points

Hi everybody, i m working on a MTSP (multiple traveling salesman problem) and because of the high number of cities.

I have coordinates xy stored in two columns (x and y), and the matrix of the distances (symmetric and hollow) and finally a criterion of distance dmax.

I would establish city groups by making as follows: If the distance between a city and a city group is lower than dmax, this city integrates the city group, and so on.

If somebody knows an algorithm of this kind, or has already worked on a similar project, get in touch with me please. Thank you