NATO said on Saturday it is upgrading its mission in the Baltic Sea with an air-defense frigate and other assets in response to drone incursions in Denmark.

Unidentified drones were observed near military installations in Denmark overnight, the Armed Forces said on Saturday, following several drone incursions near airports and critical infrastructure this week.

Copenhagen Airport, the Nordic region’s busiest, closed for several hours late on Monday as several large drones were seen in its airspace. Five smaller Danish airports, both civilian and military, were also shut temporarily in the following days.

In response, NATO will “conduct even more enhanced vigilance with new multi-domain assets in the Baltic Sea region,” it said in a statement emailed to Reuters.

The alliance said the new assets included “intelligence, surveillance, and reconnaissance platforms and at least one air-defense frigate.”

A NATO spokesperson said they would not provide details on which countries were contributing the extra assets.

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The new assets will enhance NATO’s “Baltic Sentry” mission, launched in January in response to a string of incidents in which power cables, telecom links, and gas pipelines on the Baltic Sea bottom have been damaged.

NATO countries have deployed frigates, patrol aircraft, and naval drones as part of the mission, to help protect critical infrastructure.

The alliance also launched the “Eastern Sentry” mission this month, to bolster the defense of Europe’s eastern flank in response to Russian drone incursions into Polish airspace.

It warned Russia on Tuesday that it would use “all necessary military and non-military tools” to defend itself after Estonia said on Friday that three Russian MiG-31 fighter jets violated its airspace for 12 minutes before NATO Italian fighter jets escorted them out.

Russia has disputed that Russian jets violated Estonia’s airspace and said that its drones had not planned to hit targets in Poland.

German Interior Minister Alexander Dobrindt said on Saturday that the threat from drones was “high” and that the country would take measures to defend itself.

Russia’s Foreign Minister Sergei Lavrov warned NATO and the European Union on Saturday that “any aggression against my country will be met with a decisive response.”

(Reporting by Andrius Sytas and Andrew Gray; Editing by Alison Williams)