It is believed a very large bomb is in the area and ready for use in the city, possibly in an attack on an RUC station. The IRA has shown itself willing to attack targets with massive bombs which inevitably cause great damage to surrounding homes.
The security forces are responding to this with a substantial increase in their visible security. Major roads into the city centre are now covered by roadchecks while special checkpoints have been set up on either side of many of the city's RUC bases.
The extra activity caused much traffic disruption to shoppers on Saturday and was still in place yesterday. At lunchtime nine separate checkpoints were encountered in a one-hour drive around the city. Police and troops could be seen paying particular attention to vans, which the IRA often uses to deliver large devices.
In a number of parts of the city centre, repair work is still continuing following half a dozen major bombings dating back to last December. These caused much dislocation of business and will cost many millions of pounds in compensation.
But an extra dimension of threat became evident last month with the IRA bombing of forensic science laboratories in south Belfast last month. As well as severely damaging the laboratories the 2,000lb (900kg) bomb damaged an estimated 700 homes in the predominantly Protestant area, though it caused no fatalities.
The damage to homes will again cost many millions of pounds to put right, with the worst-affected houses costing up to pounds 50,000 to repair. The lesson that can be drawn from this incident is that the IRA is quite prepared to damage hundreds of homes in non-republican areas.
This means the security forces must now think about how to guard against attacks on the many RUC stations and other potential targets which are situated in residential areas of the city.
Although these are generally heavily-fortified, a large bomb left parked outside will still cause substantial damage. Thus many of the current checkpoints consist of police officers examining vehicles before allowing them to get close to their stations.
In addition to the republican threat, the violent loyalist groups are also extremely active at the moment, particularly in north and south-east Belfast. So far this year, 65 people have died, exactly the same number as at the same point last year.