Dissident republicans based mainly in the Border areas of Armagh/Louth and Fermanagh/ Cavan, are believed to be intent on attacking a security force base in Northern Ireland using either a landmine or mortar device, according to senior security sources.
The interception of the arms shipment in Croatia last month, following months of Garda surveillance, revealed that the group had acquired "frequency hopping" transmitters. The Provisional IRA has used these to detonate roadside bombs, believing they stop the British army from blocking their signals through electronic countermeasures. .
The discovery of the transmitters indicates the dissidents are intent on carrying out radio-controlled explosions. It underlines suspicions raised by last month's attack on the Dublin-Belfast railway line in south Armagh, when the dissidents tried to detonate a bomb using a mobile telephone. The bombers are now believed to have more or less perfected the means to do this.
The railway was damaged when another device, a block of TM500 explosive primed with an electric detonator, exploded. The dissidents have an unknown amount of this safe and easy-to-use explosive, acquired in Croatia in the past two years. The explosive comes in small blocks and was originally designed as a demolition charge.
Security sources say the TM500 can be adapted for many uses, from blowing railway lines to initiating or "boosting" larger charges of home-made fertiliser-mix explosive. The explosive is not as powerful as the Semtex used by the Provisional IRA but is more versatile.
The shipment intercepted in Croatia on July 13th included seven collapsible anti-tank weapons, dozens of assault rifles, blocks of TM500 and thousands of rounds of ammunition. Police found the weapons in a lorry in a warehouse in Dobranje, a town about 40 miles from Split.
The weapons are understood to have been the latest in a series of shipments organised by a leading dissident in Newry, Co Down, and paid for by the "Real IRA" leadership in Dundalk.
It is now known the dissidents have had a presence in former Yugoslavia since 1997 when one of their leading members went there. Two other men, both members of the Continuity IRA from Co Fermanagh, helped drive the arms across Europe to Ireland. It is not known how much weaponry reached the dissidents.
All manner of weapons are easily available in former Yugoslavia. Under its communist government many towns had an arms factory, and the region is now regarded as one of the world's most active areas for arms-smuggling. The weapons-smuggling involves the Albanian and Croatian mafia, who collect weapons for shipment from any of the small Dalmatian ports around Split to Italy.
The dissident republicans have carried out 14 attacks so far this year. After a lull of more than a year after the "Real IRA" carried out the Omagh bomb atrocity in August 1998, in which 29 civilians were killed, it has spent the last year recruiting and training new members.
They have attempted two mortar attacks on military bases, at Roslea in Fermanagh and in south Armagh. They also attempted to fire a Russian-manufactured rocket at a police barracks in Co Tyrone and have planted two bombs, both of which failed to explode, at army bases in Derry.
In June they planted a bomb which also failed to explode at the Secretary of State's residence at Hillsborough, Co Down. The have set up a unit in London, which was responsible for planting a small bomb in Ealing last month and another under Hammersmith Bridge in May.
Forensic tests on the remains of the bomb that exploded under the bridge have confirmed that it was identical to a device found by gardai in a house search in Ballyfermot, Dublin, on May 25th.
In response to the increasing threat from the dissidents, the British army has stepped up helicopter activity and surveillance in south Armagh. As observation posts such as the one in Crossmaglen town square are removed, video platforms on Gazelle, Lynx and Puma helicopters are to be used over the Border area.
Security sources in the North were quoted last week as saying this increase in helicopter activity would last until the RUC was confident it could combat dissidents without the army's help.
The proposed increase in helicopter surveillance has been criticised by Sinn Fein, which said any further such military activity would have "serious implications for the whole process".
The local SDLP Assembly member, Mr John Fee, has also described helicopter activity in south Armagh as "scandalously high", adding: "Any increase in helicopter activity will antagonise people on the ground enormously."