The British government held an emergency meeting Thursday after a string of crude but potentially viable explosive devices were mailed to armed forces recruitment offices.

The devices, sent to seven offices in southeast England, bore the hallmarks of Northern Irish terror attacks, Downing Street said.

Counter-terrorism police are investigating and army bomb disposal crews were sent to assist. Sources said they could have caused injury.

Prime Minister David Cameron chaired a meeting of the government's COBRA emergencies committee to discuss the situation.

"Seven suspect packages have been identified as containing small, crude, but potentially viable devices bearing the hallmarks of Northern Ireland-related terrorism," a Downing Street spokeswoman said afterwards.

"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."

An envelope was delivered to an office in Chatham and a package was received in Reading on Tuesday. Another was found Wednesday in the army town of Aldershot.

Four more were delivered Thursday in Brighton, Canterbury, Oxford and Slough, the police's South East Counter Terrorism Unit (SECTU) said.

One of the packages was posted from the Republic of Ireland, sources said.

The devices will undergo forensic examination," said SECTU's Detective Superintendent Stan Gilmour.

"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."

A shopping centre in Slough to the west of London was temporarily evacuated, while cordons were placed close to all offices where packages were found.

"It is a necessary precaution until we know what we are dealing with," Gilmour said.

The Royal Mail postal operator said it was co-operating with the police investigation.

The Downing Street spokeswoman said the national threat level remained "under constant review".

The current threat level for Northern Ireland-related terrorism is severe within Northern Ireland -- the second-highest of the five threat levels -- meaning an attack is considered highly likely.

It is rated moderate in mainland Britain -- level four of five -- meaning an attack is considered possible, but not thought likely.

Dissident republican paramilitary groups reject the peace process in Northern Ireland and continue to carry out attacks, with the intention of destabilising the province and the power-sharing arrangements between the Protestant British and Catholic Irish communities.

Meanwhile, in an incident thought to be unrelated, a suspicious package was found during a routine vehicle search at the gates of Mildenhall air station in eastern England, one of the major United States military airbases in Europe.

The item was determined to be a home-made firework.

A spokeswoman for the base said the incident "is not thought to be terrorist-related".