Dissident republicans are inept but dangerous, Northern Ireland's chief constable has said.
Police have been mounting checkpoints across Northern Ireland
Security is being stepped up across Northern Ireland after the police issued a warning of an "increased terrorist threat".
Detectives said the public are likely to notice increased activity, including vehicle checkpoints.
Sir Hugh Orde said: "We have a general warning of a threat which increased to the point where we had to go public."
Last week, police said they feared dissident republicans were preparing a series of firebomb attacks, but fresh intelligence of attacks led to this latest warning.
Sir Hugh said dissident republican groups had failed "to mount anything that's even close to being sustained".
"When you're a small cornered animal going in one direction, when Northern Ireland and indeed the universe is going in another, you lash out," he said.
"What we are seeing is a group of disorganised but dangerous people lashing out - that's why we're concerned."
Police have not issued such a threat warning in Northern Ireland for a number of years.
They have appealed for people to be patient and co-operate with officers as well as reporting any suspicious activity.
In November, the Real IRA shot two police officers in Londonderry and Dungannon, and police fear dissidents could be planning another shooting or bomb attack.