Explosions were recorded before mysterious leaks in two Baltic Sea gas pipelines linking Russia and Europe, seismologists said Tuesday, raising suspicions of sabotage amid tensions over Moscow's war in Ukraine.

Photos taken by the Danish military showed large masses of bubbles on the surface of the water emanating from the three leaks in Sweden and Denmark's economic zones north of Poland, from 200 to 1,000 metres (656 feet to 0.62 miles) in diameter.

Polish Prime Minister Mateusz Morawiecki said that while the details of what happened were not yet known, "we see clearly that it's an act of sabotage, one that probably marks the next step of escalation of the situation in Ukraine".

Danish Prime Minister Mette Frederiksen meanwhile said it was "hard to imagine that it's accidental".

The Swedish National Seismic Network recorded two "massive releases of energy" shortly before the gas leaks near their locations off the coast of the Danish island of Bornholm, Uppsala University seismologist Peter Schmidt told AFP.

"With energy releases this big there isn't much else than a blast that could cause it," he added. "You can see that they are quite sudden. It is a very sudden energy release. It's not a slow collapse of something."

Russia said earlier that it was "extremely concerned" about the leaks. Asked by reporters whether it could be an act of sabotage, Kremlin spokesman Dmitry Peskov said that at the moment "it is impossible to exclude any options".

Ukraine, however, pointed the finger directly at Moscow, saying it was "nothing more than a terrorist attack planned by Russia and an act of aggression towards the EU".

A White House official said the United States would not speculate on the cause but was ready to support European efforts to investigate the leaks.

The Nord Stream 1 and 2 pipelines have been at the centre of geopolitical tensions in recent months as Russia cut gas supplies to Europe in suspected retaliation against Western sanctions following its invasion of Ukraine.

While the pipelines -- operated by a consortium majority-owned by Russian gas giant Gazprom -- are not currently in operation, they both still contain gas.

One of the leaks on Nord Stream 1 occurred in the Danish economic zone and the other in the Swedish economic zone, while the Nord Stream 2 leak was in the Danish economic zone.

A leak was first reported on Nord Stream 2 on Monday.

Danish climate and energy minister Dan Jorgensen confirmed the two Nord Stream 1 leaks in a statement to AFP on Tuesday.

"It is too early to say anything about the causes of the incidents," the Danish Ministry of Climate, Energy and Utilities said in a statement.

Two Danish military vessels have been dispatched to the area, while Sweden's government called an emergency meeting to be held on Tuesday evening.

Navigational warnings have been issued for a distance of five nautical miles and a flight height of 1,000 metres (3,280 feet).