Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
siege [siːdʒ] n оса́да
to be under siege быть*(impf) в оса́де
to lay siege to осажда́ть (осади́ть*perf)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.