Last week, a drone flew more than 350 miles beyond Ukraine's western border before crashing in Croatia, a NATO member country. The drone was carrying a bomb, Croatian officials said, and it's still not clear whether it belonged to Ukrainian or Russian forces.

Another drone recently entered the airspace of Romania, south of Ukraine. And on Tuesday, Ukraine’s military said it shot down a Russian drone that had reentered Ukraine through Polish airspace.

The trio of drone incidents has amplified concerns that Russia’s war in Ukraine could spill over into NATO countries, even if unintentionally, forcing the alliance to decide how to respond — if at all — to incidents that occur inside its borders.

US defense officials say the errant drones that entered NATO territory appeared to be largely inadvertent. Since the start of Russia’s invasion, the US military has established a deconfliction line with Russia to reduce the risk of miscalculation and make sure the two militaries operating so close together don’t inadvertently clash. The US has tested the line “once or twice a day,” according to a senior defense official, but so far, it hasn’t been needed.

But NATO has tried unsuccessfully to connect with Russia via a deconfliction hotline and written letters, raising concerns about Russia’s willingness to engage as the invasion of Ukraine has spread further west toward NATO territory, senior NATO military officials said on Wednesday.

“We are trying to communicate with them of course,” one of the officials told reporters in a briefing at NATO headquarters. “But it requires two [sides] to communicate.”

Russia brought the fight closer to NATO’s doorstep last weekend with precision-guided missile strikes near Lviv in western Ukraine, targeting a military training facility just 10 miles from Poland’s border. Those attacks came one day after Russian officials threatened convoys that are providing weapons to Ukraine from the West, though a senior US defense official said the facility was not being used for security shipments.

The US military has surveillance tools and sensors to help mitigate a potential escalation, including the capability to pick up radar emissions and infrared signatures of missile launches out of Russia or Belarus. US officials can then analyze the expected trajectory and try to keep an eye on it — so if it diverts, there is an understanding if it’s deliberate or accidental, defense officials said.

While the US and NATO have stopped drone surveillance flights inside Ukraine, the US military is flying surveillance drones and U-2 aircraft along the border, as well as using satellites overhead, according to the officials. NATO is routinely flying its Airborne Warning and Control System (AWACS) aircraft near Ukraine as well. Patriot air defense systems have also been deployed to Poland to aid in responding to projectiles that could enter NATO airspace.

“There’s lots and lots of drones flying around and so everybody’s kind of looking over their shoulder nervously at what’s going on,” said Tom Karako, a senior fellow at the Center for Strategic and International Studies. “Given the scale of what Russia is undertaking here, this sort of thing is not surprising. It’s one of the reasons that everyone is wired so tight right now.”

A risk for accidents

Tensions over the prospect of Russian drones or bombs spilling into NATO territory come as the Biden administration has drawn a careful line about what it’s willing to do to help Ukraine fight the Russians. The US and NATO have provided Ukraine with hundreds of millions of dollars in security assistance, including anti-air and anti-tank missiles, but the Biden administration has opposed a plan that involved the US delivering Polish fighter jets to Ukraine through a German air base, warning it would be escalatory.

US and NATO officials have also made clear they are not planning to deploy any troops to Ukraine. But amid commitments from President Joe Biden and other top officials to defend “every inch” of NATO territory, US and NATO officials are ramping up surveillance and patrols near the alliance’s border with Ukraine and in order to guard against any unintentional escalation.

“We are stepping up our vigilance, our presence, the way we monitor our airspace,” NATO’s secretary general, Jens Stoltenberg, said at a news conference this week. “We are both increasing the capabilities we have to monitor to track, but also to ensure that we are able to react if needed.”

Stoltenberg pointed to new Patriot air defense missile batteries that have been deployed along the alliance’s eastern flank. The drone incidents, he said, “highlights that with more military activities in the air, with drones and planes, there is a risk, for instance, accidents.”

“Therefore, we need to be extremely vigilant, we need to react when needed and we need to make sure that we have the communications, the line of communications also with the Russians to prevent an instance from really creating dangerous situations,” Stoltenberg said.

Retired Lt. Gen. Mark Hertling, a national security and military analyst for CNN, said drones can veer off course if a pilot loses control, and the unguided missiles that Russia uses can miss a target, with a range that raises the possibility of entering NATO territory, especially if Russian forces move further into western Ukraine.

But in any incident that might involve NATO airspace or territory, Hertling said, the key to avoiding escalation is communicating.

“The details matter, and when a NATO country is affected, we’d better get the details from Russia,” he said. “And it better be quick, because that’s an escalatory move, too.”

Eyes on the skies

The Russian drone the Ukrainian military said it shot down after coming from Polish air space had appeared to be surveilling the military training center that Russia attacked on Sunday, according to the Wall Street Journal.

Croatia’s defense minister Mario Banozic said the drone that crashed in an urban part of Zagreb flew through three NATO countries after leaving Ukrainian airspace, according to the Associated Press. While the defense minister said the drone was armed with an explosive device, Stoltenberg told reporters it appeared to be unarmed.

“There are elements that indicated it could have come from both” Ukraine and Russia, Bonozic said.

Stoltenberg said that the NATO’s air and missile defenses tracked the “the flight path of an object” that had entered Romanian airspace on Sunday, and Romanian fighter aircraft scrambled to investigate. He said NATO was reviewing both the Romanian and Croatian incidents.

The current assessment of NATO’s Supreme Allied Commander, officials said, is that “currently there is no threat to NATO as such. Not a deliberate threat by Russia. Russia is occupied for the time being with Ukraine.”

But there are of course risks, the officials added, which is why there are now discussions about moving NATO’s defensive systems further east.

“As we have now seen that Russia is prepared to use again, in the middle of Europe, military means to achieve political goals, it is worthwhile, and it will be discussed, to move forward the integrated air and missile defense system to cover the areas that are adjacent to Russia,” one of the officials said. Those areas include Belarus and potentially Ukraine, he added.

Jeff Edmonds, a senior analyst at the Center for a New American Security and former director for Russia on the National Security Council, said the risk to NATO territory will only increase as Russian forces move further west — and closer to the delivery of weapons that NATO is providing to Ukrainian forces.

“If and when they make progress toward the west, the more they’ll probably feel they have the freedom of maneuvering to engage things coming across the border,” Edmonds said of Russian forces. “One scenario here is that Russia strikes — not really caring what side of the border — as long as it hits the target, figuring they can call the bluff of the US and NATO not calling for a full-fledged war.”

Asked about Poland’s call on Wednesday to send NATO forces into Ukraine on a “peacekeeping” mission, the NATO military officials suggested such a plan would be untenable.

“We are looking at two nation states that are in a war. If they agree on a reliable and robust peace settlement, I do not necessarily see a need for a peacekeeping mission,” one of the officials said. “And if you are looking at the other version of ‘peacekeeping,’ which is actually ‘peace enforcement,’ I mean, that is war with Russia.”

“We would then have to ‘protect,'” the official explained, “and then shoot, and then kill, and then destroy.”