U.K. police have opened a counterterrorism investigation after three suspicious packages containing what appeared to be explosive devices were discovered at major transit hubs in London on Tuesday.

The packages were posted from the Republic of Ireland, according to a senior European government source. Ireland's police service confirmed it is helping with the investigation.

"The Met Police Counter Terrorism Command is treating the incidents as a linked series and is keeping an open mind regarding motives," London's police force said, declining to comment immediately on a possible Irish link.

Small improvised explosive devices were found at buildings at Heathrow Airport, London City Airport and Waterloo train station, Metropolitan Police said.

All three white postal bags that were found contained yellow bags thought by specialist police to be small improvised explosive devices. (Kirsty Wigglesworth/Associated Press)

"The packages — all A4-sized white postal bags containing yellow Jiffy bags [padded envelopes] — have been assessed by specialist officers to be small improvised explosive devices," police said in a statement. "These devices, at this early stage of the investigation, appear capable of igniting an initially small fire when opened."

The first call came at around 10 a.m. local time with a report of a suspicious package at the Compass Centre, an office building on the grounds of Heathrow, Britain's busiest airport. The package was opened by staff at the building, "causing the device to initiate," police said. "This resulted in part of the package burning."

The second call came in at approximately 11:40 a.m. British Transport Police were called to reports of a suspicious package in the mailroom at Waterloo Station. The package was not opened.

Around noon police were called to a report of a suspicious package at offices at City Aviation House at London City Airport. That package was not opened.

There were no injuries reported. Flights and train service were not affected.

Police officers secure Waterloo rail station near where a suspicious package was found. (Peter Nicholls/Reuters)

Police have advised all transport stations throughout Britain to "be vigilant" and to report any suspicious packages.

No arrests have been made, and no individual or group has claimed responsibility.

The United Kingdom is on its second-highest level of terrorism alert, with security services seeing an attack by international terrorists as "highly likely."

In 2017, five attacks in London and Manchester, including the bombing at an Ariana Grande concert, killed a total of 36 people.