Authorities in Greece have intensified investigations into how an explosive-packed drone ended up in waters off the west of the country.

An inquiry, involving specialised military teams, broadened at the weekend after bomb disposal experts detonated the unmanned device at sea.

The Greek defence minister, Nikos Dendias, said the drone, suspected to be of Ukrainian origin, almost certainly came from “a foreign state” although he stopped short of identifying the country.

“We know what it is, and we more or less know what it contains,” he said, while also attempting to allay fears that Greece lacked the military capabilities to defend itself against such emerging technology. “We have nothing to envy, we are creating the possibilities so that our homeland can equip its combat navy with the most developed drones and anti-drone systems that currently exist.”

It is believed the drone landed close to the shores of Lefkada, a popular resort island in the Ionian, when its operators lost control and the device drifted off course. It was found by a fisher in a cave last Thursday.

Alarm over its discovery deepened on Saturday after it was reported that the long-range kamikaze drone was laden with an estimated 100kg of explosives. Greek defence ministry officials refused to confirm it was loaded with munitions.

But in a new era of warfare, where cheap and deadly drones have increasingly changed the nature of combat, the incident raised questions over maritime security in an EU member state that is more vulnerable for having Europe’s longest coastline.

The political opposition was quick to accuse Athens’ centre-right government of being ill-prepared to deal with the threat posed by such weaponry.

On Sunday, Michalis Katrinis, the main opposition Pasok party’s defence spokesperson, said: “Mr Dendias has told us he ‘knows’ [all about the drone] but the Greek people are not allowed to learn anything about its origins, the purposes it served and how it was found, moving unhindered, around Lefkada.”

Its discovery, he added, had proved that Greece was in danger of being dragged into the theatre of war.

The small nationalist, pro-Russian Greek Solution party said the chance finding was evidence of a “conscious military provocation”.

Inspection of the maritime drone was taking place at a naval base on the Greek mainland where the device had been transported. Military sources, quoted in local media, said experts were examining the robot’s serial number as well as its inbuilt GPS for clues to its origin.

With specialist teams increasingly saying the device resembled a Ukrainian-made Magura V3 naval drone, the theory that its intended target was Russian oil and gas shipping in the Mediterranean was also gaining traction.

Kyiv has admitted taking aim at tankers used to circumvent sanctions in Russia’s shadow fleet.

This month Ukrainian drones took out two ships in the Black Sea, significantly stepping up its campaign against the Russian energy sector.

Equipped with satellite communication systems, V3 drones can carry explosive loads of up to 300kg. Their lethality has been upgraded by their autonomy and speed: the unmanned vehicles can travel for up to 60 hours at a top speed of about 50mph (80kmph).