Russia was behind an arson attack on an Estonian defence company in the capital Tallinn last month, authorities say.
Prime Minister Kristen Michal said the attack on Milrem Robotics - which supplies Ukraine with unmanned military vehicles - was ordered by Russian special services, but gave no details.
Moscow has been accused of launching sabotage attacks in a number of Nato countries that have been helping Ukraine since Russia launched its full-scale invasion of its neighbour in February 2022.
Russia denies such allegations and has been quick to reject the Estonian accusation, too. But EU countries have rushed to Estonia's support, adding such attacks will not deter them from helping Ukraine.
The attack on Milrem Robotics occurred on the night of 14-15 August.
Estonian Foreign Minister Margus Tsahkna said that, based on information collected by the country's Internal Security Service, "we can conclude with certainty that the arson attack... was an act of sabotage ordered by the special services of the Russian Federation".
Security service chief Margo Palloson told broadcaster ERR that Russia was "constantly planning, looking for opportunities, trying to find people who would carry out attacks", but that last month's arson attack was its first successful attack this year.
The foreign minister said the perpetrators had been arrested and Palloson said they were Latvians who had been living in Russia and taken part in previous attacks. Three suspects were arrested in Latvia and extradited to Estonia earlier this month.
Margus Tsahkna said that "this act of sabotage is part of Russia's broader campaign across Europe, aimed at intimidating us and weakening our support for Ukraine".
French President Emmanuel Macron responded to Estonia's assessment, saying "the multiplication of these hostile acts represents a dangerous escalation by Russia, which will only lead it into a dead end".
He mentioned the incident at Leipzig airport in Germany in August, in which an explosive-laden drone was found near Ukrainian cargo planes.
"Russia needs to know that it will not overcome our unity or our determination to continue supporting Ukraine," Macron added.
An EU spokesman said the 27-member bloc would not cease supporting Ukraine.
Nato chief Mark Rutte urged European governments to stay calm and continue supporting Ukraine.
"Russia continues its reckless campaign of hostile actions against Nato allies. It is trying to weaken our resolve and stop our support for Ukraine. But Russia is failing. It is failing on the battlefield in Ukraine and it is failing to undermine our unity and support for Ukraine," Rutte said.
In August alone, Russia was either accused or suspected of the Leipzig airport attack in Germany, a major fire at a warehouse of a defence firm in Bulgaria, a large explosion at a munitions factory outside Rome in Italy, an attempted arson attack on a Ukrainian-owned drone manufacturer in Slovakia, and two suspicious fires in Poland - the latest in a country which has reported several such attacks.
Hybrid warfare or hybrid attacks are loose terms that tend to describe non-military methods, for example cyber-attacks, sabotage or misinformation, designed to exert pressure and destabilise an adversary.
Estonia's internal security service, known as Kapo, said Russia was continuing to look for further opportunities for either sabotage or other hostile acts.
"Individuals recruited for such operations may be misled or manipulated and may be given false or fabricated justifications for their actions in an effort to conceal the involvement of the Russian state," Kapo head Margo Palloson said, external.
Russia always denies involvement in such attacks despite growing evidence to the contrary.
It accuses Nato countries of seeking to destabilise Russia.
On Monday, it repeated a threat to use nuclear weapons - on this occasion "to defend its territory should Nato countries attempt to isolate the Kaliningrad region from the rest of our country".
Kaliningrad - located between Poland and Lithuania on the Baltic Sea - is strategically important to Moscow because it houses the Russian Baltic Fleet and is one of Russia's only ice-free European ports.
It is not immediately clear what prompted the Russian threat, which was swiftly condemned by the Euro-Atlantic alliance.
"Nato is a defensive Alliance and none of our activities or exercises pose a risk to any part of Russia," Nato spokeswoman Allison Hart said.
"We strongly denounce the threat of force, including any irresponsible nuclear rhetoric."
