Counter-terrorism police and military bomb disposal experts are investigating after a series of crude but viable explosive devices that bore "all the hallmarks of Northern Ireland-related terrorism" were sent to army careers offices across south-east England this week.

According to detectives, four suspicious packages were discovered at army careers offices in Oxford, Brighton, Canterbury and Slough on Thursday.

A package was found in Aldershot, Hampshire, on Wednesday, while two packets were found on Tuesday at the armed forces careers office in Reading, Berkshire, and the army and RAF careers office in Chatham, Kent.

Although the devices were viable, they were crude and basic, making them unlikely to cause serious injury.

While the motive behind the campaign remains unknown – and no one has yet claimed responsibility – letter bombs have been used by dissident Irish republicans in the past. The postmark on one of the packages was from the Republic of Ireland and there were a couple of similar incidents late last year.

The news prompted David Cameron to chair a meeting of the government's Cobra committee. No details were released of attendees at Thursday's meeting, but it is likely that they included intelligence chiefs as well as government officials and senior police officers.

A Number 10 spokeswoman said: "Seven suspect packages have been identified as containing small, crude, but potentially viable devices bearing the hallmarks of Northern Ireland-related terrorism … These have now been safely dealt with by the police and bomb disposal units. Guidance has been issued to staff at all military establishments and Royal Mail asking them to be extra vigilant and to look out for any suspect packages, and the screening procedures for mail to armed forces careers offices are being reviewed. The national threat level remains under constant review."

The defence secretary, Philip Hammond, told LBC Radio: "It's a long while since we've seen Northern Ireland-related terrorist activity on the mainland, but of course for those living in Northern Ireland, this problem hasn't gone away and there has been a continuing drum beat, albeit of course at a much lower level than it was in the past, but a continuing drum beat of activity from dissident Republican organisations.

"I can assure you we haven't taken our eye off the ball. Northern Ireland remains very high on our list of security priorities and the various security authorities are very conscious of the fact that there is a continuing terrorist threat from dissident Republican organisations and that it could spill over into the UK mainland and we keep a very close watch on that."

The shadow Northern Ireland secretary, Ivan Lewis, said: "These devices bear the hallmarks of another attempt by dissidents to reverse the progress we have seen in Northern Ireland over the past 15 years. Their attempt to harm innocent people will be condemned by the people of Northern Ireland, including by those they claim to represent."

Stormont deputy first minister Martin McGuinness condemned those who continued to engage in violence. Details of the packages emerged hours after a pipe bomb exploded in a residential area of Newry in Northern Ireland. No one was injured in the blast.

Commenting on both incidents on Twitter, Sinn Fein's McGuinness said pipe bombs and letter bombs were an "attack on the peace process". He said: "Those responsible belong to the past. Their futile acts must be condemned."

As a result of the discovery of the packages, Queensmere shopping centre in Slough, Berkshire, was evacuated on Thursday, and St Giles, a busy street in central Oxford, was closed. Kent police said a cordon had been put in place on St Peter's Street in Canterbury.

Sussex police said a bomb disposal team had been called in after a suspicious package was reported in Queen's Road, Brighton, at 10.40am. The road was sealed off and nearby streets were also closed.

The south-east counter-terrorism unit (Sectu) – which covers Thames Valley, Hampshire, Surrey, Sussex and Kent – confirmed that suspect packages had been received at seven army careers offices. "The contents of the packages are suspicious in nature and will now be sent off for forensic examination," said Det Supt Stan Gilmour of Sectu.

He added: "When a suspect package is reported we have a routine response, which means we may need to evacuate the area if necessary until we can be sure it poses no threat to the public.

"While this can cause concern and disruption for local communities, it is a necessary precaution until we know what we are dealing with."

A Ministry of Defence spokeswoman said security advice to service personnel had been reiterated, adding: "We can confirm that we are aware of security incidents ongoing in Oxford, Slough and Brighton involving armed forces careers offices.

"The relevant police authorities are investigating with military EOD [explosive ordnance disposal] teams in support."

A security alert at RAF Mildenhall in Suffolk – sparked following a routine vehicle search at the base on Thursday morning – is understood to have been a false alarm and not related to the investigation at the armed forces recruitment offices.

A spokesman for one of the US Air Force squadrons based at RAF Mildenhall said a package had been discovered in a vehicle entering the facility just before 8.30am. As a result the main gate to the base was closed until 11am.

Lieutenant Colonel Kevin Parker, commander of the US 100th Civil Engineer Squadron, said: "The gate was closed at approximately 8.24am after 100th Security Forces Squadron personnel found a suspicious package during a routine vehicle inspection.

"The 100th Security Forces Squadron, Ministry of Defence and British Explosive Ordnance Disposal personnel worked together to clear the scene, and they have certified it is now safe."