It said that it had been sent to an army recruitment office, according to the Irish News.
The claims were made in a message to The Irish News, using a recognised codeword.
The Irish Republican Army (IRA) paramilitary group sought Northern Ireland's integration into the Republic of Ireland through violence.
The IRA has been cease-fire for several years but supporters and dissident groups are still active. Police are informing businesses and sectors in the United Kingdom to remain vigilant and report suspicious packages to police, with similar advice being previously sent to United Kingdom military forces.
In a joint statement, the two forces also revealed that the group is claiming to have sent five packages to various addresses in the United Kingdom, but to date only four have been recovered.
Investigators say they're keeping an open mind and are continuing their enquiries.
They also said they were investigating a claim that a fifth device is still unaccounted for.
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"Extensive advice has already been issued to relevant businesses and sectors to be vigilant for and report suspicious packages to police", the joint police statement continued.
No arrests have yet been made. "No one was injured", police added.
The following day, a further suspicious package was received at Glasgow University.
The City Aviation House package was not opened and no one was injured.
The terror threat in the United Kingdom is now characterized as "severe", meaning an attack is "highly likely", according to security officials.
Police have urged the public to remain vigilant and report anything suspicious via a confidential phone line (0800 789 321), filling in a secure form at www.gov.uk/act, or in an emergency dialling 999.
"At this time, only four devices have been recovered", they said.