BRUSSELS - NATO and the European Union said leaks in two major Russian underwater pipelines designed to ship natural gas to Germany were caused by "sabotage" and vowed to take strong action to protect critical European infrastructure.

However, their statements on September 28 stopped short of accusing anyone of being behind the incident, which caused natural gas prices in Europe to spike.

The Nord Stream 1 and Nord Stream 2 pipelines, which are owned by Kremlin-controlled Gazprom, burst in several locations in the exclusive economic zones of Denmark and Sweden.

Neither of the pipelines is currently in operation amid a standoff between Moscow and Brussels over Russia's invasion of Ukraine.

NATO Secretary-General Jens Stoltenberg said on September 28 that he discussed the "sabotage" of the pipelines at a meeting with Danish Defense Minister Morten Bodskov in Brussels.

"We addressed the protection of critical infrastructure in NATO countries," the chief of the Western military alliance that also includes most EU countries said on Twitter.

The EU issued a strong warning the same day to anybody attempting to attack the energy backbones of the 27-nation bloc.

"Any deliberate disruption of European energy infrastructure is utterly unacceptable and will be met with a robust and united response," EU foreign policy chief Josep Borrell said in a statement, echoing a warning by European Commission chief Ursula von der Leyen.

Borrell announced that the bloc would step up the protection of its energy infrastructure following the incidents.

"We will support any investigation aimed at getting full clarity on what happened and why, and will take further steps to increase our resilience in energy security," he noted.

Borrell's statements came after seismologists from Sweden and Denmark said they had recorded powerful explosions in areas near where evidence of leaks from the Nord Stream gas pipelines in the Baltic Sea had been found, prompting Swedish police to launch an investigation into possible sabotage.

"These incidents are not a coincidence and affect us all," Borrell said.

Neither NATO nor the EU accused anyone in particular of being behind the leaks, which come as Europe tries to fill up its natural gas storage ahead of winter amid the biggest energy crisis in decades.

Ukrainian presidential adviser Mikhaylo Podolyak tweeted on September 27 that the reported gas leaks were likely the result of a "terrorist attack" carried out by Moscow.

Podolyak accused Russia of seeking to "destabilize the economic situation in Europe and cause pre-winter panic."

Western officials for months have been warning that Russia could cut natural gas exports to Europe to pressure Brussels over its support of Ukraine.

Russia accounted for about 40 percent of EU natural gas supplies last year, giving it enormous leverage over energy prices in the bloc. Much of the natural gas, which is used to heat homes in the winter, flows through the Nord Stream pipelines.

Nord Stream 1 accounted for more than one-third of Russian natural gas exports to the EU last year. Nord Stream 2 was set to start operation in 2022 but Germany blocked its launch in February in a failed attempt to deter Russia from invading Ukraine.

Moscow began reducing gas flows through Nord Stream 1 earlier this year, claiming Western sanctions had caused technical difficulties, driving prices to record highs and pushing the EU toward a recession.

It completely suspended exports along the pipeline in August as the EU imposed more sanctions on Russia for its invasion of Ukraine.

Russia could face billion-dollar lawsuits from European customers if it doesn't resume the gas flows along Nord Stream 1.

But analysts say Moscow can now try to use the explosions to declare force majeure -- circumstances beyond its control due to an unforeseen event -- and avoid penalties.

The Kremlin on September 28 vehemently rejected accusations it was behind the two incidents, calling them "absurd and stupid."

It pointed the finger at Washington, adding that the United States had opposed the pipelines and that U.S. energy companies are earning large profits supplying gas to Europe.

However, prior to Russia's invasion of Ukraine, U.S. President Joe Biden declined to stop the completion of Nord Stream 2.

The Biden administration also wants to avoid a natural gas crunch in Europe this winter for fear it could weaken EU unity on Russia sanctions and support for Ukraine.

Kremlin spokesman Dmitry Peskov told reporters that the incident needed to be investigated and the timing for the repair of the damaged pipelines was not clear.

"This is a big problem for us because, firstly, both lines of Nord Stream 2 are filled with gas -- the entire system is ready to pump gas and the gas is very expensive.... Now the gas is flying off into the air."

"Are we interested in that? No, we are not. We have lost a route for gas supplies to Europe," Peskov said.

However, Russia has been burning large amounts of natural gas, a process known as flaring, for months amid much lower sales to Europe, analysts said.

Russia also has enough spare capacity along pipelines that cross Ukraine to supply Europe.

The explosions came on the same day of the inauguration of a long-awaited pipeline that will bring Norwegian gas to Poland, which used to rely heavily on Russia for supplies.

The new system will bring Norway's gas across Denmark and the Baltic Sea to Poland.

Anders Puck Nielsen, a researcher with the Center for Maritime Operations at the Royal Danish Defense College, was quoted by Reuters as saying the timing of the leaks was "conspicuous" given the ceremony for the Baltic Pipe.

He said it appeared someone may have sought "to send a signal that something could happen to the Norwegian gas."

"The arrow points in the direction of Russia," Puck Nielsen said.

"No one in the West is interested in having any kind of instability in the energy market."