PSNI Chief Constable Sir Hugh Orde has said the threat posed by dissident republicans is at its highest level since he was appointed six and a half years ago.
Mr Orde told the monthly meeting of Northern Ireland's Policing Board that it is very clear that dissidents are determined to kill police officers.
He also said co-operation between the PSNI and gardaí is superb.
He said groups of dissidents are trying to use the border to their advantage and that he is in no doubt that officers in his force are alive because of actions by An Garda Síochána.
Mr Orde is to meet Garda Commissioner Fachtna Murphy next week.
Dissidents are being blamed for making the 300lb home-made bomb that was discovered in Castlewellan Co Down last month.
It is thought that the device was intended for use at the British Army base at nearby Ballykinlar.