Gardaí and the PSNI are investigating possible links between a bomb blast in south Armagh yesterday and an on-going investigation into dissident republican activity in Co Louth.
No one was injured yesterday when a bridge was blown up in the border area of Carrickavaddy, near Newtownhamilton.
Meanwhile, gardaí in Co Louth are still questioning five men on suspicion of being members of the Real IRA.
The men, who are aged between 21 and 60 and who were arrested in Dundalk and north Louth, can be detained until Tuesay.
Northern Ireland Policing Board member Basil McCrea said the upsurge in dissident activity was worrying and said it was vital to step up efforts on both sides of the border to counteract the threat.