Three ships with Russian links have emerged as possibly playing a rolein the mysterious drone flights that closed a number of Denmark's airports several times this week. The vessels were spotted off the Danish coast around the time of Wednesday's incident and may have served as launch pads for the drones raising troubling questions about the scope of Russias hybrid warfare in the Baltic Sea region.

Suspected drone sightings led Danish authorities to brieflyclosean airportagain on Friday, days after drones caused airports in Copenhagen and the Norwegian capital Oslo to shut down. Drones were spotted over airports inseveral otherDanish cities later in the week.

Prime Minister Mette Frederiksen has said called the incidents the "most serious attack" on Danish infrastructure to date, describing them as "hybrid attacks" with possible Russian links.

Three maritime vessels the Astrol-1, the Pushpa and the Oslo Carrier-3 aresuspected of playing a rolein the suspicious drone flights that closedCopenhagenairport on Tuesday.

Data relating to the presence of ships in the area from wheredronesmay have taken off and landed is part of the investigation, Copenhagen police confirmed to FRANCE 24, adding that the probe is a multi-pronged effort involving the police, the security services, the army, and other national and supranational authorities.

Two cargo ships and an oil tanker

Air traffic was brought to a standstill starting Monday evening in both Copenhagen and Oslo, with the airports reopening several hours later.A new wave of drones were spotted flying overfour more Danish airportsovernight into Thursday.The airport in the northern city of Aalborg was forced to close for several hours, with flightssuspended briefly againovernight into Friday.

"This is part of the development we have recently observed with other drone attacks, airspace violations, andcyberattackstargeting European airports," Frederiksen said in a statement, referring to a string of incidents inPolandandEstoniarecently and whereRussiais also suspected of involvement.

Establishing the drones point of departure could be key in identifying those behind the incursions. Danish media has flagged three vessels of particular interest and all of them have some link to Russia.Open-source investigatorshave identified them as two cargo vessels and one oil tanker.

Read moreRussian fighter jets enter NATO member Estonias airspace in brazen incursion

Danish TV2 journalist Peter Moller, who tracked the ships movements online, said that the Astrol-1, for example, followed an an erratic trajectory with a large number of zigzags before entering the waters near Copenhagen.

Alexander Lott, a maritime law and security expert at the Arctic University of Norway, said that anything that is out of the ordinary is suspicious, because a ship has no reason not to take the shortest route to get from one point to another.

So if it circles around, or travels in zigzags, it can raise suspicions.

Using ships as launch pads?

The 117-metre-long vessel known as Astrol-1 sails under the Russian flag. It left the northern Russian city of Arkhangelsk for Saint Petersburg last week, passing through the Oresund Strait near Copenhagen the same day the first drone flights took place.

At approximately the same time the drones were first spotted on Monday, the Oslo Carrier-3 was located in waters just 7 kilometres from the airport.

This proximity has drawn suspicion, despite its tenuous links to Russia. Some of the Oslo Carrier-3 crew are Russian-speakers, and its owner,Bulkship Management AS, runs a recruitment office in Kaliningrad.

The third suspect vessel, the Pushpa oil tanker, has the strongest ties to Russia. Although it sails under the Benin flag, it has been accused of transporting Russian oil as part of Moscows sanctions-evadingshadow fleet. Pushpa also sailed near Copenhagen at around 8:30pm on Monday but on the other side of Zealand island and south of the Danish capital.

Experts say there are plenty of advantages in using ships as platforms for drone launches. It is relatively easy for the operators of such drones to conceal or get rid of any sort of proof of their involvement, Lott said.

Julian Pawlak, a Baltic and maritime security specialist at Bundeswehr University Hamburg, noted that ships also have the advantage of being mobile. So you can move around you can actually hide."

Without a mobile platform, a Russian drone operation would have had to be launched either from a Russian territory like Kaliningrad, or from Danish or European soil. "But then they would have needed to hide, and of course this is also hard, Pawlak said.

Just how difficult is it to launch drones from ships? It depends on the type of drone used, said Bruno Oliveira Martins, an expert on emerging technology security at the Peace Research Institute Oslo. Size matters, he explained, noting that smaller drones that take off vertically can be launched from virtually anywhere, even from small boats.

But larger drones, that have fixed wings and so on, need larger [platforms] to take off.

Aiming for airports

Even if the ships in question were not used as launch pads, Oliveira Martins said they may have been involved in other ways, serving, for example, as floating command centres for the drone operators. If this turns out to be the case, he said, it's an audacious, even ingenious operation.

"Drones are known to be a major problem for airports, he noted, and by bringing them onboard vessels, they can be brought much closer to their targets than they would have been otherwise.

Airports are also particularly difficult to protect from drone activity. Detecting is not always easy, Oliveira Martins said, adding that although airports are equipped with radars, these are also used to direct flight traffic and to detect other risks, like birds.

Another big challenge is how to take the drones down. The entities that manage the airports don't have the authority to use kinetic systems to bring drones down," he noted. "And jamming, for example, is very problematic because it also interferes with other radio frequencies ... such as that between the control tower and the airplanes.

TheBaltic Searegion has become the focal point of tensions between Russia andNATOcountries, Pawlak said, citingthesuspected sabotageof undersea cables by Russias shadow fleet. But with drones now possibly entering the mix, the situation has become even more explosive.

"There is no bulletproof way to defend against the threat of drones," he said.

One of the main problems is that it is almost impossible to prevent ships even those flying under the Russian flag from entering national waters. This is due to the maritime law of the right of innocent passage, which prevents national authorities or the coast guard from interfering with passing ships unless they have reasonable grounds to suspect the vessel is being used to commit a crime.

Proving that a vessel has been used for hostile drone operations is very difficult, Lott said. Unless they are caught red-handed.

Even if Denmark were to conclude the drones were launched from one of the three suspect vessels, it cannot automatically accuse Russia.

When it comes to planes, its easy to say that a state is behind the operation. But in relation to drones you have the whole problem of the potential involvement of non-state actors, possibly but not necessarily acting on behalf of a state, Lott said.

In other words, involvement becomes much easier to deny.

Oliveira Martins said that drones have become the perfect platform for conducting hybrid warfare: They are difficult to spot, and this is very good when you want to carry out covert operations below the threshold of war.

"As we've seen in Ukraine," he added, they can also become a deadly weapon in an instant.

This article was adapted from theoriginalin French by Louise Nordstrom.

Originally published on France24