The Baltic Sea pipeline and a telecommunications cable connecting Finland and Estonia were damaged on Sunday.
On Tuesday, Helsinki said the damage was likely caused by "outside activity" and that the cause was being investigated.
"It can clearly be seen that these damages are caused by quite heavy force," Estonian Defence Minister Hanno Pevkur told Reuters, adding that investigators were not ruling out anything at this stage.
"So what it is exactly, we have to specify yet, but at the moment it rather seems that it had been mechanical impact or mechanical destruction."
Henri Vanhanen, research fellow at the Finnish Institute for International Affairs, said the central issue would be how NATO reacted if evidence was gathered that a state actor was behind the pipeline damage. Finland and Estonia are both members.
"Because this could mean that this is an attack against NATO member infrastructure," he said. "I think the big question in the long term is that do we have a clear set of potential countermeasures for such activities? What is the deterrence?"
NATO Secretary-General Jens Stoltenberg, heading into a meeting of alliance defence ministers to discuss the war in Ukraine, said if the damage was proven to be due to an attack, it would be met by a "united and determined" response.
The incident occurred a little over a year after the larger Nord Stream gas pipelines, which cross the Baltic Sea between Russia and Germany, were damaged by explosions that authorities have said were deliberate acts of sabotage.
The gas pipeline and the telecommunications cable run parallel but at a "significant" distance from each other, according to the telecommunications cable operator, Elisa.
The damage to the pipeline was believed to have taken place in Finnish waters, while the cable breach was in Estonian waters, Finnish authorities have said.
The pipeline, which was encrusted in concrete for protection, looks like "someone tore it on the side", according to Estonian Navy Commander Juri Saska.
"The concrete has broken, or peeled off, specifically at that point of injury," Saska told Estonian public broadcaster ERR.
"It looks like it was pulled from one side, like a watering hose stuck behind your leg and you've dragged it behind you."
The Kremlin described news of the damage as disturbing and said it was awaiting further information on the incident.
Kremlin spokesman Dmitry Peskov, speaking at a regular news briefing, noted that Russia's Nord Stream pipelines had also been damaged by an attack in the Baltic Sea.
