MiG-31 fighter jets are not commonly seen close to Europe but some were intercepted along with other aircraft above the Baltic Sea in two separate incidents on Tuesday and Wednesday. (Ilya Naymushin/Reuters)

NATO said Wednesday that it had intercepted a large number of Russian aircraft flying close to European airspace in the past two days, in an “unusual” series of incidents that brought Russian bombers as far afield as Portugal.

The aircraft — at least 19 in all — offered reminders of Russian air power at a time of the worst relations between the West and Russia since the Cold War. Russian military aircraft have significantly increased their activity in Europe since the conflict in Ukraine began earlier this year, with NATO scrambling to intercept aircraft more than 100 times in 2014. But a NATO official said the scale of the latest incidents was the most provocative this year.

Over the Atlantic Ocean and the North, Black and Baltic seas, Russian bombers, fighter jets and tanker aircraft were detected flying in international airspace, NATO said. There were no incursions into national airspace, a violation of sovereignty that would have significantly amplified the seriousness of the four incidents, three of which took place on Wednesday.

“We’re raising it as an unusual level of activity,” said Lt. Col. Jay Janzen, a spokesman for NATO’s military command in Mons, Belgium. “The flights we’ve seen in the last 24 hours, the size of those flights and some of the flight plans are definitely unusual.”

U.S. officials regard the flights as a show of force by the Putin government. “It’s concerning because it’s moving in the wrong direction,” said one U.S. defense official, speaking on the condition of anonymity because he was not authorized to discuss the air activity publicly. “It’s not helping to de-escalate the situation in Ukraine. It’s not helping to improve relations between NATO and Russia. It’s not helping anybody.”

At least nineteen Russian aircraft, including fighter jets and bombers, have been intercepted in four waves over Europe yesterday and today.

Smaller-scale incidents have also increased this year, approximately tripling from the same period in 2013, Janzen said.

In at least one of the four incidents, the aircraft had switched off their transponders and had not filed flight plans with civilian air traffic controllers. That means that civilian air traffic control cannot track them, potentially creating a risk for civilian planes.

That incident took place around 3:00 a.m. in Western Europe on Wednesday, when four Tu-95 long-range strategic nuclear bombers and four Il-78 tanker aircraft flew over the Norwegian Sea. Norwegian F-16 fighter jets scrambled to intercept them. Six of the planes returned to Russia, but two of the bombers skirted the Norwegian coast, flew past Britain — sending Typhoon fighter jets to scramble in response — and then finally looped west of Spain and Portugal, attracting Portuguese F-16s. Then the two bombers appeared to return to Russia, Janzen said.

The Tu-95 bombers are not commonly seen close to Europe, Janzen said. Nor are the MiG-31 fighter jets that were intercepted along with other aircraft above the Baltic Sea in two separate incidents Tuesday and Wednesday. It was not immediately clear whether the two incidents above the Baltic represented the same group of seven planes entering and departing a Russian military base at Kaliningrad.

There was no immediate reaction from the Russian government.

Fighter jets from Norway, Britain, Portugal, Turkey, Germany, Denmark, Finland and Sweden were involved in responding to the Russian aircraft, Janzen said. Finland and Sweden are not members of NATO, and they have long refrained from joining the defensive alliance, which was formed after World War II as a bulwark against the Soviet Union.

But military incidents with Russia this year have caused both countries to start to reevaluate their positions. Most recently, the Swedish military last week spent several days searching a vast territory for an unidentified underwater craft suspected to be Russian. Last month, Sweden said two Russian military planes had violated its airspace.

A Novus opinion poll released Tuesday found for the first time that more Swedes favored joining NATO than opposed it.

The most recent violation of NATO airspace was last week, when a Russian spy plane flew almost 2,000 feet into Estonian airspace. This year, NATO increased its air patrols based in the Baltics from four to 16 jets, a measure of the newly hot confrontation between the two military juggernauts.

The incidents appear to have set European militaries on edge this week. British fighter jets were scrambled Wednesday to bring a civilian Antonov cargo jet into a London airport; it stopped responding to radio calls from air traffic controllers while flying over the British capital. That caused a supersonic boom that was audible across a large stretch of southeastern England.

Missy Ryan contributed to this report.