The Swedish navy encounters Russian submarines in the Baltic Sea on an “almost weekly” basis, its chief of operations has said, and is preparing for a further increase in the event of ceasefire or armistice in the Ukraine war.

Capt Marko Petkovic said Moscow was “continuously reinforcing” its presence in the region, and sightings of its vessels were a regular part of life for the Swedish navy. Its “very common”, he said, adding that the number of sightings had increased in recent years.

The Baltic Sea region is facing an increasing range of threats, including suspected hybrid attacks from drones, alleged sabotage of underwater infrastructure and a steady flow of ageing oil tankers in the form of shadow fleet ships carrying crude oil from Russia.

Last month, the British defence secretary said that a Russian spy ship had entered British waters and shone lasers at military pilots, warning that the UK faced a “new era of threat” from hostile countries.

Sweden recently hosted a major Nato anti-submarine warfare exercise, Playbook Merlin 25, featuring nine countries, including Sweden, Germany, France and the US, in which hundreds of personnel practised their submarine-hunting skills in the unique conditions of the Baltic to prepare for possible underwater attack.

The hilly underwater landscape of the Baltic near Sweden makes it difficult to detect submarines because they can hide.

Petkovic said Russia was increasing its capabilities and was producing one Kilo-class submarine a year in St Petersburg and the Kaliningrad enclave, positioned between Poland and Lithuania. He said it was undergoing a “deliberate and constant modernisation programme” of its ships.

“Once a ceasefire or armistice is eventually in place in Ukraine, you can only assess, and we do assess that Russia will reinforce its capabilities in this region,” said Petkovic. “So with that said, the [Swedish] navy needs to continuously grow and focus on the overall picture.”

He said Russia’s shadow fleet of civilian-flagged oil tankers was also a concern and did not rule out the potential for such ships to be used to launch drones.

“The shadow fleet in itself is not a military problem, but the shadow fleet could affect our nations from a military perspective,” Petkovic said.

The varying challenges of underwater conditions – including shorter visibility than above water, salinity and temperature – mean that underwater infrastructure was especially vulnerable in the Baltics, he said. This was especially the case for Sweden, Norway, Finland, Estonia and Lithuania, he said, which were “fully dependent on the sea lines for communication, for our sustainment of our societies”.

However, he believes that increased Nato vigilance is having an impact. He said that since Operation Baltic Sentry was established in January, “we haven’t seen any cable incidents in this region at all”.

He added: “First of all it shows that the alliance works, cohesion. And we are closing ranks against one particular threat. The Baltic Sentry has proven that point. Regardless of whether any of the incidents have been state-sponsored or if it’s been bad seamanship, or anything in between, it has raised awareness amongst the merchant fleet that they should be a bit more cautious when travelling our regional waters.”