Senior police sources have told UTV the 'New IRA' sent the devices to armed forces offices in Oxford, Reading, Chatham, Aldershot and Canterbury.A total of seven have been mailed altogether - four were intercepted on Thursday in addition to three which were found earlier this week.UTV correspondent Sharon O'Neill said: "Like all letterbombs they are very crude in nature but can still cause injury. The problem with them in terms of finding those responsible is they're hard to trace."But it is emerging tonight that two of them came from the Republic. Were any posted from here?We don't know yet - but what we do know is that the PSNI and An Garda Síochána are working closely with police forces in England."The A4 envelopes were discovered by Army officers, but it is not clear whether the packages could have exploded. A shopping centre in Slough also received one of the potentially explosive parcels.A Number 10 spokeswoman described them as "crude", but said injuries could have been caused.She added that they bore "hallmarks of Northern Ireland-related terrorism". And she said the level of national terrorist threat in Great Britain - currently set at 'moderate', while NI is assessed to be 'severe' - remains under constant review.@PoliceFedforNIhave consistently warned that dissident republicans would target GB when it suited their perverse agenda!— PFNI (@PoliceFedforNI) February 13, 2014Staff at all military establishments are said to have been warned to be vigilant, while Royal Mail has also been briefed on the issue.Prime Minister David Cameron has also chaired a meeting of the Cobra emergency committee to discuss the packages.Intelligence chiefs as well as government officials and senior police officers attended the briefing.Detective Superintendant Stan Gilmore, of the South East counter-terrorism unit, said the devices are believed to pose a "low-level" threat."The contents of the packages are suspicious in nature and will now be sent off for forensic examination," he explained."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."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."Political representatives have condemned the attacks.Pipe bombs &letter bombs are an attack on the Peace Process.Those responsible belong to the past,their futile acts must be condemned. #Peace— Martin McGuinness (@M_McGuinness_SF) February 13, 2014Nigel Dodds, DUP MP, said: "It is by God's grace that no one has been injured by these crude devices. Those behind the deadly packages are to be condemned. Those who cling to terrorism should realise that it failed in the past and it will do so again."It will only lead to further hurt and suffering. Northern Ireland has turned a corner. We are moving forward and no one wants to go back to the bad old days."UUP leader Mike Nesbitt said: "We must all be thankful that the letter bombs were dealt with before they caused death or injury."My thoughts are with the servicemen and women who continue to go about their duties defending this country despite the threat from those who hide behind masks and skulk in the shadows."Shadow Secretary of State Ivan Lewis said: "Their attempt to harm innocent people will be condemned by the people of Northern Ireland, including by those they claim to represent."Stewart Dickson MLA, Alliance, said: "I would like to express my gratitude to the Police and Army officers who put their lives at risk to deal with these letter bombs.CHAR(13) + CHAR(10)CHAR(13) + CHAR(10)"I would urge anybody with any information about these devices to contact the police."