Russia denied violating Estonian airspace.Estonia has requested a consultation with other Nato members after Russian warplanes violated its airspace on Friday morning.
Three Russian MiG-31 fighter jets entered the Estonian skies "without permission and remained there for a total of 12 minutes" over the Gulf of Finland, the government said.
Italy, Finland and Sweden scrambled jets under Nato's mission to bolster its eastern flank. A Nato spokesperson said it was "yet another example of reckless Russian behaviour and Nato's ability to respond".
Russia denied violating Estonian airspace. But tensions have been escalating recently, after Poland and Romania - both Nato members - said Russian drones breached their airspace.
Article 4 of the Nato treaty formally starts urgent consultations within the 32-member alliance, which ties the US and many European nations together on collective defence.
It is the second time this month that a Nato member has requested Article 4 consultations. Poland did so on 10 September after Russian drones entered its airspace.
Estonian Prime Minister Kristen Michal said "Nato's response to any provocation must be united and strong".
"We consider it essential to consult with our allies to ensure shared situational awareness and to agree on our next joint steps," Michal said.
US President Donald Trump said on Friday: "I don't love it. I don't like when that happens. Could be big trouble."
Estonia, which shares a border with Russia to the east, says this was the fifth Russian violation of its airspace this year.
Officials said the Russian aircraft entered its airspace from the north-east and were intercepted by Finnish jets over the Gulf of Finland. Once inside Estonian airspace, Italian F-35 jets, based in Estonia, were deployed under Nato's Baltic Air Policing mission to escort the aircraft out.
The government said the Russian jets had no flight plans, had their transponders turned off and also did not have two-way radio communication with Estonian air traffic control.
Russia's defence ministry said the jets were on a "scheduled flight... in strict compliance with international airspace regulations and did not violate the borders of other states, as confirmed by objective monitoring".
It said they flew over neutral Baltic waters, more than 3km (two miles) from Vaindloo Island, which belongs to Estonia.
President Trump has been leading efforts to end the conflict - most recently by inviting his Russian counterpart Vladimir Putin to Alaska for a summit.
But Putin's forces have repeatedly rejected calls for a ceasefire. They have intensified air attacks, and have been making slow progress on the ground - despite very high combat casualties reported.
Michal said the Russian incursion showed its war of aggression in Ukraine was not proceeding as the Kremlin had planned.
"The aim is to draw attention and assistance away from Ukraine by forcing Nato countries to focus more on the defence of their own territories," he added.
Friday's incursion was not an isolated incident, even if it was considered the most serious violation of Estonian airspace so far.
Speaking to the BBC on Saturday morning, Estonian Foreign Minister Margus Tsahkna said it was part of a pattern of Russian behaviour.
He used the analogy of the boiling frog to explain Russia's strategy. The suggestion is that by very gradually ramping up provocations, none of which on their own would necessitate a military response, Russia is able to test and ultimately undermine Nato's defences.
That is why there are calls, including from former UK Defence Secretary Sir Ben Wallace, for the alliance to take a much tougher line with the Kremlin.
He argued Russia needed to be presented with a "strategic dilemma" - a response that would demonstrate its provocations came with a cost.
The question is, what would that be? In 2015, a much briefer violation of airspace over Turkey - another Nato member - ended with the shooting down of a Russian jet and the eventual death of the pilot.
That resulted in diplomatic and economic consequences, but not all-out war. And no similar incursion has been reported since.
Would the same be true if the MiGs had been shot down this time? It's impossible to say with any certainty.
One vital added complication would be the response of Donald Trump.
As ever with this president, it is a fool's game to make firm predictions - but it is very possible he would view the shooting down of Russian planes, even in Nato airspace, as a significant and unnecessary escalation - and one he would oppose.
Were a wider conflict to result, it is entirely conceivable he would view Europe as being significantly responsible and so may refuse to offer vital US support.
That would have repercussions beyond a single incident – calling into question Nato's much vaunted Article 5, which says that an armed attack against one or more members will be considered an attack against all.
That would be a nightmare scenario for Europe - and a dream one for the Kremlin. All in all, it has created an environment of extreme strategic uncertainly for European powers, one they are still trying to decide how to respond to.
Last week, Poland's military said it had shot down at least three Russian drones, with prime minister Donald Tusk saying 19 drones were recorded entering Polish airspace.
Russia insisted the incident was not deliberate, and its defence ministry said there had been "no plans" to target facilities on Polish soil.
Belarus, a close Russian ally, said the drones entered Polish airspace accidentally after their navigation systems were jammed.
Several days later, Romania's defence ministry said it had detected a Russian drone when two F-16 jets were monitoring the country's border with Ukraine, after "Russian air attacks on Ukrainian infrastructure on the Danube [river]".
The ministry said the drone later disappeared from the radar.
Russia has not commented on the issue.
In response to Russia's incursions into Poland and Romania, Nato pledged to move troops and fighter jets eastwards.
Planes from the UK, France, Germany and Denmark are all taking part in air defence missions over Poland in a bid to bolster the alliance's eastern flank.
