Nato has promised a “determined” response if damage to an undersea gas pipeline between Finland and Estonia proves deliberate, as investigators said traces of an “external, mechanical force” had been found on the seabed. Amid widespread media speculation about the likelihood of Russian sabotage, Risto Lohi of the Finnish national bureau of investigation told a press conference in Helsinki on Wednesday: “There is reason to suspect an external force … caused the damage.” The force, he added, “appears to have been mechanical, not an explosion”. The agency’s chief, Robin Lardot, said marks had been found on the seabed at the site of the damage to the Balticconnector pipeline. Its operators said it would take at least five months to repair the pipeline, meaning it was unlikely to come on stream again until April 2024. But Lardot said the investigation into aggravated vandalism was in its “very early technical stages” and could take several more days because of poor weather and the large search area. The bureau was working to “find facts and analyse them”, he said. Finnish authorities announced on Tuesday that a sudden fall in pressure in the pipeline recorded on Sunday had been caused by extensive damage that “appeared to be deliberate”. Nato’s secretary general, Jens Stoltenberg, said on Wednesday that if the damage to the Balticconnector was “proven to be an attack on Nato critical infrastructure … it will be met by a united and determined response from Nato”. The incident happened just over a year after a series of underwater blasts burst three of the four pipelines that make up Nord Stream 1 and Nord Stream 2, a major conduit for Russian fossil gas exports to western Europe, spewing gas into the Baltic Sea. Finland’s preparedness level was raised on Wednesday after an extraordinary meeting of a high-level ministerial committee on foreign and security policy, but the security of the country’s energy supply – of which gas accounts for just 5% – was rated stable. Speaking at a Nato meeting in Brussels, Finland’s defence minister, Antti Häkkänen, said Helsinki “values ​​the support offered by its allies” in the transatlantic military partnership, but refused to speculate on the outcome of the investigation. The Finnish foreign minister, Elina Valtonen, said no decision would be made on what action should be taken until the investigation was completed and “we have all possible information”. Estonia’s defence minister, Hanno Pevkur, said investigators “must be allowed to get on with their work”. Commander Jüri Saska of the Estonian navy told ERR public radio that Finnish pictures of the damage shared with the Tallinn government showed the Balticconnector’s concrete protective cover had been broken or torn off, and the pipeline itself was out of position and badly damaged on one side. A Finnish expert, Jukka Savolainen of the 22-member European Centre of Excellence for Countering Hybrid Threats (Hybrid CoE), told the Finnish public broadcaster YLE that one possibility was that “a large ship” had dragged its anchor over the pipeline – either intentionally or, in stormy weather, unintentionally. Finnish media reported on Wednesday that five large vessels, including a Panama-flagged freighter and four oil tankers, one of them Russian, were near the 48-mile (77km) pipeline, which runs across the seabed of the Gulf of Finland from Inkoo in Finland to Paldiski in Estonia, shortly before the incident. After Norwegian seismologists said on Tuesday they had recorded a “thud” at about 1.20am on Sunday in the vicinity of the pipeline, the University of Helsinki’s seismology institute said its instruments had also recorded a “weak vibration” at about the same time. Analysts have said a Russian hydrographic survey vessel, Sibiryakov, was detected in the Gulf of Finland near the pipeline and the Estlink communications cable, which was also severed, on at least three occasions in May, August and September. The Kremlin spokesperson, Dmitry Peskov, on Wednesday described the incident as “disturbing”, adding that he did “not have any technical information, and do not know if our special services have any such information”. Commissioned in 2019, the Balticconnector has been the only gas import channel to Finland – apart from liquefied natural gas (LNG) – since Russian imports were halted last May as part of sanctions imposed against Moscow after its invasion of Ukraine.