An assistant chief constable of the Police Service of Northern Ireland Will Kerr has said there is evidence that dissident republicans are attempting to develop armour-penetrating devices.

The devices are similar to those used to carry out attacks on military vehicles in Iraq and Afghanistan.

Mr Kerr, who has overall responsibility for the PSNI's crime operations, also said that intelligence-gathering and surveillance are being used to counteract the threat posed by dissidents on both sides of the border.

He said the gardaí has been immensely supportive in tackling these gangs.

The Assistant Chief Constable said dissidents have been attempting to develop and use sophisticated Improvised Explosive Devices.

He cited the examples of explosively formed projectiles (EFP's), used in attacks on PSNI vehicles in Belfast and Derry in recent months.

The internet carries many examples of such devices used to penetrate armoured military vehicles and kill the occupants in Afghanistan and Iraq.

In the Derry and Belfast incidents, the devices did not work as intended but they were used in built-up areas.

Separately, British army bomb experts have made safe a viable device sent to PSNI headquarters in Belfast.

The alert was raised after a suspicious package in a brown padded bag with a white address label was received through the post.

Superintendent Sam Donaldson said someone could have been seriously hurt.

"This is a mindless and foolish act that could have resulted in the serious injury of any person handling the package," he said.

"Although we have no information to suggest there may be other similar packages in the postal system, I would encourage people to be vigilant."

Last year a group calling itself the IRA said it had sent several letter bombs to army recruitment offices across the UK.

This marked the resurgence of a terror tactic adopted by paramilitaries during the Troubles.

Four suspected explosive devices were discovered at Army careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough.

Packets were also sent to military careers offices in Aldershot in Hampshire, Reading in Berkshire and Chatham in Kent.

In 2013, dissident republicans were blamed for sending a series of letter bombs to high-profile political and security figures in Northern Ireland.