Eighty officers are currently involved in searches for bomb-making material that the PSNI believes is being stored by the 'New IRA' in Creggan.
Superintendent Gordon McCalmont said 80 officers, made up of specialist search officers and neighbourhood officers supported by explosive detection dogs, were involved in the searches on Monday morning.
He was speaking after a thwarted mortar bomb attack in Strabane at the weekend which police also believe was the work of the 'New IRA.'
"Our assessment is that this group is concealing bomb-making materials in this area.
"We have a number of areas of focus, however there is no definitive and no guarantees. However, our chances are improved by support and information from the community," said Superintendent McCalmont.