Investigators are looking for a possible unexploded letter bomb after an Irish terror group said it had sent five packages to targets across Britain last week, three of which were discovered in London and one in Glasgow.

In a claim of responsibility, a group calling itself the IRA said it was behind the packages and added that a fifth letter containing an improvised explosive device had been addressed to an unnamed army recruitment centre.

The Guardian understands investigators believe the group is actually the one known to the police and security services as the New IRA, which has previously focused its activities on the Derry area, where it bombed a court building in January.

The police and security services are trying to understand the group’s tactics in sending devices to British targets. Its rhetoric in the past has railed against “crown forces personnel and their imperial establishment”.

Last week, a package was sent to the army recruitment office at Glasgow University and three others to commercial targets close to transport hubs across London. One device partially ignited but no one was injured. The three others were made safe.

Police are trying to find the alleged fifth device and have advised postal sorting offices and army personnel to be vigilant.

Investigators doubt the devices were intended to kill or cause serious harm, and the terrorism threat level relating to Irish groups has been kept at “moderate”, meaning an attack is considered possible but not likely.

Police Scotland and the Metropolitan police said in a joint statement that the claim was received on Monday by the Belfast-based Irish News using a recognised codeword.

“Those claiming responsibility have indicated five devices were sent. At this time, only four devices have been recovered. Extensive advice has already been issued to relevant businesses and sectors to be vigilant for and report suspicious packages to police,” they said.

“This advice was previously sent to armed forces personnel and is being reiterated again in light of this claim. We continue to urge the public to remain vigilant and report anything suspicious to police.”

Even before the claim of responsibility, police were exploring the possibility of dissident Irish republican involvement, as the packages bore similarities to devices sent in the past. No arrests have been made.

Detectives from the Met’s counter-terrorism command are leading the investigation into the London packages and Police Scotland are leading the investigation into the Glasgow package.

Homemade devices were found in the post room at London Waterloo station, City Aviation House at London City airport, and the Compass Centre, close to the grounds of Heathrow, last Tuesday.

All three packages had stamps from the Irish Republic and had Dublin as the return address. The devices were in A4-size yellow Jiffy bags contained within white postal bags.

The package at the Compass Centre was the only one opened. It ignited, resulting in part of the package burning.

On Wednesday, bomb disposal experts carried out a controlled explosion of a device found in the main campus mailroom at Glasgow University. The package, targeting army recruitment on the campus, had not been opened and again, no one was injured.