The PSNI commander in Foyle has said there is a deteriorating security situation in the area.
Commander Chris Yates said the threat in the district was "severe", and had been for "a long time" .
"Severe means that an attack is highly likely and the figures bear that out," he said.
"In 2008/09 there were 28 terrorists offences within G District. Last year there was 106. We are on an upward slope."
He added: "Last year we had about 15 pipe bombs in the district, most of them would have been across Foyle. This year, to date, we are up to 16.
"We are on a par this year with what we saw last year, and last year was a particularly bad year."
There have been five security alerts in the north west in the past week because of pipe bombs and Commander Yates said he was concerned people were not taking the threat of them seriously enough.
"A pipe bomb is the equivalent of a hand grenade. These things are being left in residential streets. We are very lucky that nobody has been killed or seriously injured," he said.
"These things are very, very dangerous."
He also appealed to people in the areas affected to cooperate with police and pass on any information about those planting the pipe bombs.
"We do know who they are, but there is a world of difference between knowing who they are and having the evidence you need to put them before the court.
"There is a mountain of forensic evidence at the lab and we do anticipate there will be further arrests."