Ukraine's military intelligence has said that a recent drone attack on an airport in northwestern Russia which damaged several transport planes was carried out from within Russian territory.
The claim, falling on the first day of the school year, came as President Vladimir Putin told Russian students that their country was "invincible" and police in the Ukrainian capital Kyiv responded to bomb threats in schools.
The attack earlier this week on Pskov airport - 700km from Ukraine - came after Kyiv vowed to "return" the conflict to Moscow in July.
"The drones used to attack the 'Kresty' air base in Pskov were launched from Russia," Ukraine's intelligence chief Kyrylo Budanov said on social media.
"Four Russian IL-76 military transport planes were hit as a result of the attack. Two were destroyed and two were seriously damaged," he added.
Mr Budanov said the aircraft had been used by the defence ministry to transport troops and cargo.
Russia said this week that military experts were working to find out which routes the drones were taking in order "to prevent such situations in the future".
Asked about the Ukrainian claims, Kremlin spokesman Dmitry Peskov declined to comment and instead deferred questions to the defence ministry.
The region of Pskov, which was also targeted by drones in late May, is surrounded by NATO members Estonia and Latvia to its west and Belarus to its south.
Mr Budanov's comments came hours after Russian air defences destroyed a drone approaching Moscow, the city's mayor said, a day after a similar attack on the capital.
Russian media reported that air traffic at Moscow's Domodedovo and Vnukovo airports had been temporarily halted.
An increase in aerial assaults on the capital's financial district, damaged commercial buildings and even struck the Kremlin but officials have dismissed the rise in attacks.
Ukrainian forces have been making slow gains against Russian positions, in particular in southern Ukraine, since launching a counteroffensive in June.
Kyiv, which has been criticised for the pace of the gains, has said it is not under pressure from the West to move more quickly and hailed its recent capture of the village of Robotnye as a gateway to the annexed Crimean peninsula.
The United States said that Ukrainian forces have, in recent days, made "notable progress" against Russian troops in their southern offensive.
"Any objective observer of this counteroffensive, you can't deny ... that they have made progress now," White House national security spokesman John Kirby told reporters, describing criticism of the Ukrainian effort by anonymous officials as "not helpful".
Putin to host Erdogan on Monday
Alongside the increase in drone attacks inside Russia, tensions have been building on the Black Sea after Moscow in July scrapped a deal allowing maritime exports from Ukraine.
But Ukraine has established an alternate route for cargo vessels and announced today that two more vessels had departed, defying a Russian naval blockade.
Turkey, which brokered the deal allowing grain exports from Ukraine with the United Nations, has urged Moscow to return to the accord.
The Kremlin announced today that President Putin will host his Turkish counterpart Recep Tayyip Erdogan in the Black Sea resort of Sochi for talks on Monday, likely on the scuppered agreement.
During a meeting with Turkish Foreign Minister Hakan Fidan in Moscow, Russian Defence Minister Sergei Shoigu blamed the West for the collapse of the grain deal, saying Russia's demands had not been met to save the pact.
"But it turned out that it is more difficult to do this than to build new corridors and new land routes," Mr Shoigu said, referring to Ukraine's new maritime routes on the Black Sea.
