POLICE Service of Northern Ireland (PSNI) chiefs have expressed concern after Semtex explosives were used in a dissident Republican rocket attack against police officers in the North.On Tuesday the dissident Republican group the Continuity IRA claimed responsibility for a rocket attack on police officers in the Fermanagh town of Lisnaskea.Two officers were treated for shock while a third was treated for minor injuries after the device was fired at their foot patrol late on Saturday night.However, while the device failed to explode police chiefs have expressed concern that dissidents used semtex in the attack.More than two tons of the powerful explosive are thought to have been smuggled into Ireland from Libya by the IRA in the 1980s.Throughout the 1980s and 1990s the IRA used semtex as its favored explosive.However, when dissidents split from the IRA in the late 1990s they are thought to have brought some of the plastic explosives with them.Despite Gardai recovering semtex in Real IRA bomb factories in 1999 and 2002 dissidents have rarely, if ever, used it.The use of semtex was only discovered because the bombers dropped the rocket launcher at the scene when the device failed to fire.The attempted bomb attack comes just days after Chief Constable Hugh Orde raised concerns that dissidents were planning to kill police officers.Warning of the increased threat from dissidents after the weekend attack, Deputy Chief Constable Paul Leighton said, "We've had seven officers now in this area who have been attacked over the past number of months - that's seven potential deaths. Society wants to move on."We all have visions of the future - we may have slightly different visions of how that turns out - but the one thing that's common in those visions is that violence plays no part in it."