Nato will mount a “determined” response if damage to the Balticconnector undersea gas pipeline, which runs between member states Finland and Estonia, is found to be deliberate, Nato secretary-general Jens Stolenberg said on Wednesday.

The pipeline was closed on Sunday after a suspected leak caused a sharp drop in pressure. On Tuesday, damage was officially confirmed by Finland’s state-owned gas system operator Gasgrid, which also said that repairs will take more than five months, with the pipeline expected to become operational again in April next year at the earliest.

If damage to the pipeline is “proven to be a deliberate attack on Nato-critical infrastructure then this will be, of course, serious, but it will also be met by a united and determined response from Nato,” Stolenberg told reporters in Brussels ahead of a meeting of the military alliance.

The announcement from Nato follows widespread speculation over the likelihood of Russian sabotage, which came after Risto Lohi, chief inspector of the Finnish national bureau of investigation (NBI), said on Wednesday at a press conference in Helsinki that “there is reason to suspect an external force… caused the damage.” The force, he added, “appears to have been mechanical, not an explosion”.

NBI director Robin Lardot also said that marks had been found on the seabed at the site of the damage to the Balticconnector, the Guardian reports, although he added that the investigation into possible vandalism is in its “very early technical stages”. It could take several more days to reach a formal conclusion due to poor weather conditions and the vast area being evaluated.

Theories that a large ship or tanker had either deliberately or accidentally dragged an anchor through the pipeline have emerged in the Finnish media. Estonian Defence Minister Hanno Pevkur told Reuters that damage to the Balticconector had “clearly” been caused by “quite heavy force”.

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By GlobalData

Estonian Navy Commander Juri Saska told Estonia’s public broadcaster, ERR, that the pipeline looks as though “someone tore it on the side”, adding that the concrete that encases it, for protection, “has broken, or peeled off, specifically at that point of injury”.

Finland’s Foreign Minister, Elina Valtonen, said that no decision will be made on a response until the investigation is completed and “we have all possible information”. The country’s Defence Minister Antti Häkkänen also refused to speculate on the outcome of the investigation.

Speculation of Russian involvement could come from previous detections of a Russian hydrographic survey vessel, named the Sibiryakov, in the Gulf of Finland close to the pipeline, the latest of which occurred in September, according to analyst reports.

A spokesperson for the Kremlin, Dmitry Peskov, on Wednesday called the incident “disturbing”, adding that he did “not have any technical information, and does not know if our special services have any such information”.

While Estonia has been a member of Nato for almost two decades, Finland only joined the alliance earlier this year.