A group calling itself the IRA has claimed responsibility for sending suspect packages to British army recruitment centres in England last week.
Scotland Yard said a group using a recognised codeword had contacted a media outlet in Northern Ireland at the weekend.
Seven suspected explosive devices were sent to army careers offices.
One of the packages bore a Republic of Ireland postmark and the British government said the small but potentially viable devices bore "the hallmarks of Northern Ireland-related terrorism".
Scotland Yard said: "We are aware of the claim of responsibility for the devices that were sent to Army recruitment centres in England last week.
"The claim was received on Saturday February 15 by a Northern Irish media outlet using a recognised codeword. The claim was allegedly made on behalf of the 'IRA'.
"The public is urged to remain vigilant and report anything suspicious to the Anti-Terrorist Hotline, 0800 789 321."
The IRA disbanded in the years after it declared an end to its armed campaign in 2005.
However, a group calling itself the New IRA formed just before the London Olympics in 2012 and was linked to letter bombs sent last autumn.