Russian Navy ships and submarines were observed close to the Nord Stream pipelines where the explosions took place, Western intelligence sources have said.
European security officials saw the movements by the Russian Navy on Monday and Tuesday, as well as last week.
But it doesn’t necessarily mean they were involved in suspected sabotage of the pipeline from Russia to European neighbours, said the intelligence sources, reported CNN.
The Nord Stream pipelines have been flashpoints in an escalating energy war between capitals in Europe and Moscow that has damaged major Western economies and sent gas prices soaring.
As gas spewed out under the Baltic Sea for a third day after first being detected, it remained far from clear who might be responsible for any sabotage of the pipelines that Russia and European partners spent billions of dollars building.
And a Danish military official also is understood to have said that Russian Navy ships often operate in the area.
“We see them every week. Russian activities in the Baltic Sea have increased in recent years. They’re quite often testing our awareness – both at sea and in the air.," the person told CNN.
Amid suspicion towards Russia for the apparent sabotage of the pipeline, as they have the capability and motivation to carry out the attack, so far there is no proof.
"The jury is still out," a US official said, briefing Pentagon reporters on the condition of anonymity. "Many of our partners, I think, have determined or believe it is sabotage.
"I'm just - I'm not at the point where I can tell you one way or the other."
The White House was cautious when asked about potential retaliation but acknowledged the concerns stemming from any attack on critical infrastructure of NATO allies.
"I don't want to get ahead of the investigation," said White House spokesperson Karine Jean-Pierre.
Both Denmark and Sweden are carrying out investigations into the leaks but so far there is little detail over what happened and it could take some time before any conclusions are reached. Even the safety of the site is uncertain for an investigation to begin, due to a possible build up of pressure in the pipes.
Asked whether the Pentagon, perhaps utilizing submarine or other under-sea military capabilities, was assisting with the investigation, the US military official said Washington had not been asked to provide that support.
"We're like a number of other countries out there with capability that could certainly assist, but we haven't been asked to do so," the official said. "And again, there are a lot of countries out there that have underwater capability."
US Defence Secretary Lloyd Austin also offered support in a call with his Danish counterpart on Wednesday, a senior US defence official said, also speaking on condition of anonymity.
Navy Captain Tamara Lawrence, a spokesperson for US Naval Forces Europe-Africa, said the US Navy stood ready to provide support and assistance "in close coordination with our allies and partners, if needed."
The European Union on Wednesday promised a "robust" response to any intentional disruption of its energy infrastructure after saying it suspected sabotage was behind the gas leaks discovered this week on subsea Russian pipelines to Europe.
Russia reduced gas supplies to Europe via Nord Stream 1 before suspending flows altogether in August, blaming Western sanctions for causing technical difficulties. European politicians say that was a pretext to stop supplying gas.
The new Nord Stream 2 pipeline had yet to enter commercial operations. The plan to use it to supply gas was scrapped by Germany days before Russia began what it calls a "special military operation" in Ukraine in late February.
Russia's embassy in Denmark said any sabotage on Nord Stream's pipelines was an attack on both Russia's and Europe's energy security.
