OSLO—Recent Russian military activity in European airspace is illegal, provocative and dangerous, said U.K. Defense Secretary Michael Fallon Thursday, as a group of Northern European defense ministers met to address an increase in Russian military activity.

“We’ve seen larger [Russian] formations, going further,” Mr. Fallon told The Wall Street Journal in an interview. “On Oct .31, we saw Russian aircraft go past Norway, past Denmark, past the U.K., past Ireland, all the way down to Portugal. Portugal and Ireland are of no concern to Russia.”

Described the recent Russian aircraft activity as unnecessary, Mr. Fallon said “it’s illegal and doesn't conform to the conventions of international aviation. It’s provocative, it’s intimidating and frankly it’s dangerous.”

Russian aircraft that don’t file flight plans or that maintain no radio contact are posing a risk to civil aviation, according to the North Atlantic Treaty Organization. NATO has recorded more than 100 intercepts of Russian aircraft in European airspace so far this year, three times the total last year.

Asked whether the Russians were increasing their activity to test Western defense capabilities, Mr. Fallon said: “if they are, we are responding.” The 12 countries meeting in Oslo Thursday discussed closer cooperation on the sharing of data and intelligence on the Russian airspace activity, he added.