European countries say they will ramp up security around oil and gas installations after the suspected sabotage of two major pipelines.
The EU, US and Nato have suggested damage to the pipelines between Russia and Germany was deliberate, but have not blamed Russia directly.
Russia has said it was not involved, and asked if the US was instead.
Russia has previously been accused of using gas supplies as a weapon against the West over its support for Ukraine.
The leaks on the Nord Stream 1 and Nord Stream 2 pipelines were discovered on Monday and Tuesday.
Danish Energy Minister Dan Jorgensen said the leaks were likely to last for at least a week, until the gas escaping from the pipes runs out. An investigation has been launched.
European Commission head Ursula von der Leyen has promised the “strongest possible response” if the attack is proven to have been deliberate.
President Vladimir Putin’s spokesman Dmitry Peskov has dismissed accusations of sabotage as “predictable, stupid and absurd”.