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. Finnish authorities announced on Tuesday a sudden fall in pressure in the pipeline on Sunday – 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. Amid widespread media speculation about the likelihood of Russian sabotage, Finnish prime minister Petteri Orpo said Finland had launched an investigation and damage to the pipeline was likely the result of "external activity"