BRUSSELS—The crossing of Russian drones into the airspace of NATO countries is emerging as a test of the alliance’s red lines and its support to Ukraine in repelling Russian invaders without triggering a wider war.
Ukraine’s military said Tuesday it downed a Russian drone as the unmanned aerial vehicle returned after crossing into Polish airspace. Russia hasn’t commented on the incident. Earlier this week, another drone suspected of belonging to the Russian military crashed in Romania. Allied officials have said they are studying the incident.
“We are very closely monitoring airspace and the border areas around NATO,” Jens Stoltenberg, the head of the alliance, said Tuesday. “Our military commanders also have lines to the Russian commanders to help prevent incidents and accidents, and to help prevent them from spiraling out of control if they happen.”
The incidents come as the North Atlantic Treaty Organization is on high alert for Russian incursions into alliance territory. NATO defense ministers are meeting Wednesday to discuss plans to reinforce forces in member countries bordering Ukraine, Russia and Belarus.
The U.S. has deployed Patriot air-defense batteries to Poland and Germany shipped some to Slovakia, among other moves to reinforce the alliance’s eastern flank. More than 100 combat jets have been deployed along NATO’s eastern border close to Russia. U.S. Secretary of State Antony Blinken and other NATO leaders have repeatedly said they would protect “every inch” of alliance territory.
“There’s always the risk of incidents and accidents,” Mr. Stoltenberg said ahead of Wednesday’s meeting. “Therefore we have to make every effort to prevent such incidents and accidents, and if they happen, to make sure they don’t spiral out of control and create really dangerous situations.”
Mr. Stoltenberg declined to comment on the alleged Russian incursion into NATO airspace. Poland’s Defense Ministry declined to comment beyond saying it was “monitoring the situation and taking necessary measures to ensure the security of the country.”
The drone appeared to be surveilling a Ukrainian military training center close to the Polish border that was struck by Russian missiles on Sunday, killing at least 35 people, Ukraine’s military said.
Russian drones are increasingly conducting reconnaissance near the Ukrainian border, as well as that between Poland and Belarus, a senior Western intelligence official said. “We have reliable indications of this, based on the position and range of drones we see.”
Romania’s response to the drone crash on its territory also has been muted. A Defense Ministry official said civilian authorities are investigating the area where the drone went down.
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The Western intelligence official said the drone crash in Romania suggests Russia is attempting to conduct reconnaissance in the far-west Ukraine. The crash came shortly after Russian officials issued warnings about Western countries providing military support to Kyiv that passes through western Ukraine.
If Russian drones entered NATO airspace, they may have been testing the alliance’s air defenses. They also could have been badly piloted or had navigation problems. A senior Western diplomat said there was no indication that Russia has been trying to trigger incidents on NATO territory.
“We have not seen any attempt to engage allied forces—quite the opposite,” said the diplomat, who added that the drones may have entered NATO airspace due to navigation problems or other faults in their design or production.
Mr. Stoltenberg said NATO forces tracked the flight path of an object that entered Romanian airspace on Sunday. In response, Romanian fighter aircraft scrambled immediately to investigate, and the alliance is reviewing the incident.
U.S. Ambassador to NATO Julianne Smith said she wouldn’t parse each scenario that could lead an alliance member to invoke the group’s most foundational principle, Article 5 of its founding treaty, which enshrines that an armed attack on one represents an attack on all. “We are all prepared to come to the aid of a country, should they feel the need to invoke Article 5,” she said.
A surveillance drone entering NATO airspace isn’t an armed attack, the senior Western diplomat said.
Spy drones may pose a particular challenge in defining when Russia has crossed a red line. They often aren’t armed and don’t pose a direct lethal threat. The information they gather, though, can be instrumental in planning an attack.
Days before the Polish and Romanian incidents, another drone, suspected to be of Ukrainian origin, crashed in Croatia, prompting the government there to ask the French military to conduct a surveillance flight of its airspace. That flight showed nothing suspicious, the French military said.
Mr. Stoltenberg said NATO believes the drone wasn’t armed, but the incident highlights that with all those drones and planes in the aircraft “there is more risk of incidents and accidents and therefore we need to be extremely vigilant.”
Defending against drone incursions, even by relatively unsophisticated models, also poses a challenge. NATO’s high-end air-defense systems are optimized to shoot down fighter and bomber planes or even ballistic missiles, not relatively slow, low-flying drones.
“Drones are really small, they are maneuverable, they fly low to the ground. As a result it is hard to identify them and hard to track them,” said Arthur Holland Michel, a senior fellow at the Carnegie Council.
Many of the antidrone systems that are being deployed by militaries are relatively short-range and designed to protect specific facilities, he said, not expansive borders such as those separating NATO from Russian forces.
—Isabel Coles contributed to this article.
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