The British Foreign Office summoned the Russian ambassador in London on Monday to protest what it called Moscow's "significant and unprecedented violation of NATO airspace."

"Significant and unprecedented violation of Polish and NATO airspace by Russian drones last week – followed by a further incursion into Romanian airspace on Saturday – was utterly unacceptable," a Foreign Office spokesperson said in a statement.

The spokesperson said the UK stands united with Poland, Romania, Ukraine, and its NATO allies in "unreservedly condemning these reckless actions."

The statement said that the defense of Ukraine against Russia is "crucial" to the security of the whole of Europe, including the UK.

"Russia should understand that its continued aggression only strengthens the unity between NATO allies and our determination to stand with Ukraine, and any further incursions will again be met with force," it added.

Romania's Defense Ministry said Saturday that a Russian drone entered its airspace during an attack on Ukraine, the latest in a series of cross-border incidents along NATO’s eastern flank.

Last week, Poland said at least 19 drones crossed into Polish airspace when Russia was attacking Ukraine. Russia, however, said the drone incursion was not deliberate.

In response, NATO announced new measures to strengthen defenses with the launch of Operation Eastern Sentry.

Moscow denied responsibility for the incident in Poland and called the drone incursion reported by Romania a "provocation" by Ukraine.(AA)