HELSINKI (Reuters) – Finland is watching its territorial waters very closely following explosions of two Nord Stream gas pipelines, Prime Minister Sanna Marin said on Wednesday.

Marin told reporters that Finland, which shares the European Union’s longest land border with Russia, considers the explosions “grave news” and “extremely worrying”. The pipelines were built by Russia and European partners.

The European Union on Wednesday promised a “robust” response to any intentional disruption of its energy infrastructure after saying it suspected sabotage was behind gas leaks under the Baltic Sea. Russia has also said sabotage was a possibility.

“Finnish authorities and the government is watching the situation very closely. We are monitoring the situation in the Finnish economic area and our territorial waters,” Marin said.

Defence minister Antti Kaikkonen said Finland had not detected increased military activity in its vicinity in recent days but keeps close contact with Nordic neighbours and NATO allies to exchange information.

The government said it will move to restrict heavily the transit of Russian citizens holding tourist visas to other countries in the Schengen region via Finland, refusing to give a date of enforcement before an official decision over the matter expected on Thursday.

Finnish land border crossings have remained among the few entry points into Europe for Russians after a string of Western countries shut both physical frontiers and their air space to Russian planes in response to Russia’s invasion of Ukraine seven months ago.

Finland will also consider building a fence on parts of its 1,300-km border with Russia, with an estimated cost of several hundreds of millions of euros, minister of interior Krista Mikkonen said. The fence would take 3-4 years to build, she said.

(Reporting by Anne Kauranen; Editing by Grant McCool)