(CNN) European security officials on Monday and Tuesday observed Russian Navy support ships in the vicinity of leaks in the Nord Stream pipelines likely caused by underwater explosions, according two Western intelligence officials and one other source familiar with the matter.
It's unclear whether the ships had anything to do with those explosions, these sources and others said -- but it's one of the many factors that investigators will be looking into.
Russian submarines were also observed not far from those areas last week, one of the intelligence officials said.
Three US officials said that the US has no thorough explanation yet for what happened, days after the explosions appeared to cause three separate and simultaneous leaks in the two pipelines on Monday.
Russian ships routinely operate in the area, according to one Danish military official, who emphasized that the presence of the ships doesn't necessarily indicate that Russia caused the damage.
"We see them every week," this person said. "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."
But the sightings still cast further suspicion on Russia, which has drawn the most attention from both European and US officials as the only actor in the region believed to have both the capability and motivation to deliberately damage the pipelines.
US officials declined to comment on the intelligence about the ships on Wednesday.
Both Denmark and Sweden are investigating, but a site inspection has yet to be done and details on exactly what caused the explosions remains sketchy. One European official said that there is a Danish government assessment underway and it could take up to two weeks for an investigation to properly begin because the pressure in the pipes makes it difficult to approach the site of the leaks — although another source familiar with the matter said the probe could begin as soon as Sunday.
The prime ministers for both Denmark and Sweden said publicly on Tuesday that the leaks were likely the result of deliberate actions, not accidents, and Sweden's security service said in a statement Wednesday that it cannot be ruled out "that a foreign power is behind it." US national security adviser Jake Sullivan on Tuesday evening also called the leaks "apparent sabotage" in a tweet.
But senior Western officials have so far stopped short of attributing the attack to Russia or any other nation.
The Kremlin has publicly denied striking the pipelines. A spokesman called the allegation "predictably stupid and absurd."
CNN has reached out to the Russian Ministry of Defense for comment on the presence of the ships.
The Danish government is taking the lead on the investigation and has put in place an exclusion area of five nautical miles and a 1 kilometer no-fly-zone, according to European sources familiar with the matter.
Other than Sullivan, US officials have been far more circumspect than their European counterparts in drawing conclusions about the leaks.
"I think many of our partners have determined or believe it is sabotage. I'm not at the point where I can tell you one way or the other," a senior military official said Wednesday. "The only thing I know there is that we think the water is between 80 and 100 meters [deep] at that location where the pipeline is. Other than that, I don't know anything more."
But one senior US official and a US military official both said Russia is still the leading suspect -- assuming that the European assessment of deliberate sabotage is borne out -- because there are no other plausible suspects with the ability and will to carry out the operation.
"It's hard to imagine any other actor in the region with the capabilities and interest to carry out such an operation," the Danish military official said.
Russia has requested a UN Security Council meeting on the damaged pipeline this week -- something the senior US official said is also suspicious. Typically, the official said, Russia isn't organized enough to move so quickly, suggesting that the maneuver was pre-planned.
If Russia did deliberately cause the explosions, it would be effectively sabotaging its own pipelines: Russian state company Gazprom is the majority shareholder in Nord Stream 1 and the sole owner of Nord Stream 2.
But officials familiar with the latest intelligence say that Moscow would likely view such a step as worth the price if it helped raise the costs of supporting Ukraine for Europe. US and western intelligence officials believe Russian President Vladimir Putin is gambling that as electricity costs rise and winter approaches, European publics could turn against the Western strategy of isolating Russia economically. Sabotaging the pipelines could "show what Russia is capable of," one US official said.
Russia has already taken steps to manipulate energy flows in ways that caused itself economic pain, but also hurt Europe. Russia slashed 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.
"They've already shown they're perfectly happy to do that," one of the sources said. "They weight their economic pain against Europe's."
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 sent troops into Ukraine in February.
US, European and Ukrainian officials have been warning for months, however, that critical infrastructure -- not only in Ukraine but also in the US and Europe -- could be targeted by Russia as part of its war on Ukraine.
The US warned several European allies over the summer, including Germany, that the Nord Stream 1 and 2 pipelines could face threats and even be attacked, according to two people familiar with the intelligence and the warnings.
The warnings were based on US intelligence assessments, but they were vague, the people said -- it was not clear from the warnings who might be responsible for any attacks on the pipelines or when they might occur.
The CIA declined to comment.
Der Spiegel was the first to report on the intelligence warnings.