A counter-terror operation is continuing after several suspected explosive devices said to bear the "hallmarks of Northern Ireland-related terrorism" were sent to military sites in England.

No specific group has claimed responsibility, but one of the packages was stamped with a Republic of Ireland postmark.

The packages were found at armed forces careers offices in Oxford, Slough and Brighton yesterday, the South East Counter Terrorism Unit (Sectu) said.

Another package was found in a vehicle which was stopped and searched near RAF Mildenhall in Suffolk, which hosts the US military.

The discoveries come after the army and post offices were put on alert after two "basic but viable explosive devices" were found earlier this week by military recruiters.

"Seven suspect packages have been identified as containing small, crude, but potentially viable devices bearing the hallmarks of Northern Ireland-related terrorism," a No. 10 spokesman said after a meeting of the Government's Cobra emergencies committee was held to discuss the packages.

"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 is being reviewed."

Sky’s Ireland correspondent David Blevins said: "We know that police on this side of the Irish Sea have been very concerned for some time about the growing capability of dissident Republicans - renegade groupings who oppose the peace settlement here."

Political leaders in Northern Ireland have condemned those behind the attacks.

Among them was Ulster Unionist leader Mike Nesbitt, who said: "This was the work of cowards."

Northern Ireland First Minister Martin McGuinness said on Twitter: "Pipe bombs & letter bombs are an attack on the Peace Process. Those responsible belong in the past, their futile acts must be condemned."

St Giles', which runs adjacent to Oxford colleges including Keble and Trinity, was closed while bomb disposal experts carried out their work.

Slough's Queensmere Shopping Centre and part of Brighton town centre were evacuated while the packages were investigated.

Officers also set up a cordon at St Peters Street in Canterbury, Kent, where another suspicious package was believed to have been reported at an army careers office.

On Tuesday a package sent in the post to an army recruiting office in Reading, Berkshire, was found to contain a "small but viable" explosive device.

Similar packages were discovered in Chatham on Tuesday and in Aldershot on Wednesday.

Detective Superintendent Stan Gilmour, of Sectu, said the contents of the packages were "suspicious in nature" and would be forensically examined.

He said: "Even if the contents are determined to be a viable device, they pose a very low-level threat and are unlikely to cause significant harm or damage."

Sky’s Home Affairs correspondent Mark White said: "The advice to the military and Royal Mail is to look out for anything that looks at all suspicious.

"The national threat level remains the same, but the Government says the terror threat level remains under constant review."

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