Involved in ds DNA break repair. Has a role in non-homologous integration (NHI) pathways where it is required in the final step of non-homologous end-joining. ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. Belongs to the ATP-dependent DNA ligase family.