The Swedish coast guard seized a Bulgarian ship after a fibre-optic cable under the Baltic Sea linking Sweden to Latvia was damaged, officials said on Monday.

Latvia sent a warship on Sunday to investigate the damage, while Swedish prosecutors opened an "aggravated sabotage" investigation.

Nations around the Baltic Sea have scrambled to bolster defences after the suspected sabotage of undersea cables in recent months, with some observers blaming Russia.

The Swedish coast guard on Sunday instructed the Bulgarian ship Vezhen "to go into their territorial waters and it is now on anchor where an investigation... is ongoing," Alexander Kalchev, CEO of Navigation Maritime Bulgare (Navibulgar), the ship's operator, told AFP.

Images of the vessel published by Swedish media appeared to show a broken arm on one of Vezhen's anchors.

Kalchev denied involvement in any sabotage. "I am convinced that we cannot say... that this was a malicious act," he added.

The Malta-flagged vessel carrying fertilizer from Ust-Luga in Russia and headed for South America was sailing in "extremely bad weather" on Saturday based on the information given to him by the crew, Kalchev said.

An inspection on Sunday found that "one of the ship's anchors was damaged and the anchor had dropped into the sea, which means that it was possible that it had dragged along the seafloor," he said, adding that the anchor was then pulled up.

The Swedish prosecutor in charge of the case, Mats Ljungqvist, told AFP that "the ship suspected of aggravated sabotage" was currently located off the coast of Karlskrona, near the small island of Aspo, but would not confirm the vessel's name.

According to marine traffic websites, the Vezhen is anchored at that location, with two Swedish coast guard vessels next to it.

"Staff from several Swedish authorities" have been on board the vessel since Sunday evening as part of the investigation, Ljungqvist said.

He said that the the case was "primarily being handled by" the Sapo intelligence agency.

Navibulgar, Bulgaria's biggest shipping company, has appointed an agent in Sweden and hired a lawyer "to defend the interests of the crew and the company", which extends its "full support" in the probe, according to Kalchev.

The ship -- constructed in 2022 -- has a crew of eight Bulgarians and nine Myanmar nationals.

Kalchev confirmed media reports that the ship was leased from Chinese company Hai Kuo Shipping, but said that it "has no control over the ship in any way".

The damage to the cable occurred in Swedish territorial waters at a depth of at least 50 metres (164 feet), officials said.

The cable belongs to Latvia's state radio and television centre (LVRTC) which said in a statement on Sunday that there had been "disruptions in data transmission services".

The company said that alternatives had been found and end users would mostly not be affected, although "there may be delays in data transmission speeds".

Prime Minister Evika Silina said on Sunday that Riga had notified the Swedish authorities and that the two countries were working together on the incident.

Swedish Prime Minister Ulf Kristersson confirmed on Sunday that he had been in contact with Silina, and said that Sweden, Latvia and NATO were closely cooperating on the matter.

Experts and politicians have accused Russia of orchestrating a hybrid war against the West as the two sides square off over Ukraine.

NATO this month announced it was launching a new monitoring mission in the Baltic Sea involving ships and aircraft to deter attempts to target undersea infrastructure.

European Union President Ursula von der Leyen expressed "full solidarity" with countries affected by the incident.

"The resilience and security of our critical infrastructure is a top priority," von der Leyen wrote on X.