Security for police officers is now under review after a booby trap device exploded underneath the car of an officer who was travelling along the Drumnabey Road in Spamount village near Castlederg on Monday evening.

It is understood that business owners in the city have also been warned by the PSNI to step up security on their premises because of a heightened dissident threat.

The police officer, whose name has not been released, was on his way to start a night shift in Enniskillen when the device exploded.

He suffered serious leg injuries from the attack and is currently recovering in hospital after undergoing surgery.

This week's attack is the second attack on an off-duty police officer in the North West within the space of a year. In November PSNI officer Jim Doherty was shot by dissident republicans as he dropped his child off at school in Bishop Street in the city.

In a visit to Derry yesterday, assistant chief constable Judith Gillespie said the PSNI had been expecting further dissident attacks on the police and on business premises.

"We are at a very early stage and cannot say which particular group we suspect," she said. "Although dissident republican activity is the main line of enquiry. We have been warning for some time that the capacity for dissident attacks on the police and on business premises exists.

"We are talking to various sources who are working closely with us to develop this investigation. We are concerned but we do not want a return to policing from behind barriers. We will not be deterred from providing a proper police service to all of the community."

The assistant chief constable said that security for off-duty police officers would be reviewed in light of the attack.

"We always advise our officers to take sensible precautions when off-duty. We are always reviewing the security situation and that will continue. We will be discussing the implications of this attack at the top level of the PSNI and with the Policing Federation. We will not be deterred by these attacks."