Drones struck inside Russia’s border with Ukraine Tuesday in the second day of attacks exposing the vulnerability of some of Moscow’s most important military sites, observers said.

Ukrainian officials did not formally confirm carrying out drone strikes inside Russia, and they have maintained ambiguity over previous high-profile attacks.

But Ukrainian presidential adviser Mikhail Podolyak taunted Moscow in comments on Twitter, and Britain’s Defense Ministry said Russia was likely to consider the attacks on Russian bases more than 500 kilometers (300 miles) from the border with Ukraine as “some of the most strategically significant failures of force protection since its invasion of Ukraine.”

Kremlin spokesman Dmitry Peskov said that Russian authorities will “take the necessary measures” to enhance protection of key facilities. Russian bloggers who generally maintain contacts with officials in their country’s military criticized the lack of defensive measures.

“If something is launched into other countries’ airspace, sooner or later unknown flying objects will return to the point of departure,” Ukrainian presidential adviser Mikhail Podolyak taunted Moscow in comments on Twitter. “The earth is round.”

A fire broke out at an airport in Russia’s southern Kursk region that borders Ukraine after a drone hit the facility, the region’s governor said Tuesday. In a second incident, an industrial plant 80 kilometers (50 miles) from the Ukrainian border was also targeted by drones, which missed a fuel depot at the site, Russian independent media reported.

“They will have less aviation equipment after being damaged due to these mysterious explosions,” said Yurii Ihnat, spokesman for the Air Force Command of the Armed Forces of Ukraine. “This is undoubtedly excellent news because if one or two aircraft fail, then in the future, some more aircraft may fail in some way. This reduces their capabilities.”

Moscow blamed Kyiv for unprecedented attacks on two air bases deep inside Russia on Monday. The attacks on the Engels base in the Saratov region on the Volga River and the Dyagilevo base in the Ryazan region in western Russia were some of the most brazen inside Russia during the war.

In the aftermath, Russian troops carried out another wave of missile strikes on Ukrainian territory struck homes and buildings and killed civilians, compounding damage done to power and other infrastructure over weeks of missile attacks.

Approximately half of households in the Kyiv region remain without electricity, the regional governor said Tuesday, while authorities in the southern Odesa say they have managed to restore power to hospitals and some vital services.

In a new display of defiance from Kyiv, Ukrainian President Volodymyr Zelenskyy traveled to an eastern city near the front line. Marking Ukraine’s Armed Forces Day, Zelenskyy traveled to the eastern Donetsk region and vowed to push Russian forces out of all of Ukraine’s territory.

“Everyone sees your strength and your skill. … I’m grateful to your parents. They raised real heroes,” Zelenskyy said Tuesday in a video address to Ukrainian forces from the city of Sloviansk, a key Ukrainian stronghold in the east.

The Tu-141 Strizh (Swift) drone entered service with the Soviet air force in the 1970s and was designed for reconnaissance duties. It can be fitted with a warhead that effectively turns into a cruise missile.

Unlike modern drones, it can only stay in the air for a limited amount of time and fly straight to its designated target.

Its outdated technology makes it easily detectable by modern air defense systems and easy to shoot down.

Another Soviet-built drone in the Ukrainian armed forces’ inventory, the Tu-143 Reis (Flight) has a much shorter range of about 180 kilometers (about 110 miles).

A Russian pro-war blogger posting on the Telegram channel “Milinfolive” on Monday hit out at Russian military leadership, alleging that incompetence and lack of proper fortifications at the airbases made Ukrainian drone strikes possible.

Russia’s Defense Ministry said three Russian servicemen were killed and four others wounded by debris, and that two aircraft were slightly damaged.

Pro-Kremlin political analyst Sergei Markov said the strikes “have raised questions about security of Russian military air bases.”

The Engels base hosts Tu-95 and Tu-160 nuclear-capable strategic bombers that have been involved in strikes on Ukraine. Dyagilevo houses tanker aircraft used for mid-air refueling.

In a daily intelligence update on the war in Ukraine, Britain’s Defense Ministry said the bombers would likely be dispersed to other airfields.

Speaking in a conference call with reporters Tuesday, Peskov said that “the Ukrainian regime’s course for continuation of such terror attacks poses a threat.”

Peskov reaffirmed that Russia sees no prospects for peace talks now, adding that “the Russian Federation must achieve its stated goals.”

Russia, meanwhile, maintained intense attacks on Ukrainian territory, shelling towns overnight near the Zaporizhzhia nuclear power plant that left more than 9,000 homes without running water, local Ukrainian officials said.

The towns lie across the Dnieper River from the nuclear plant, which was seized by Russian forces in the early stages of the war. Russia and Ukraine have for months accused each other of shelling at and around the plant.

The head of Ukraine’s northern Sumy region, which borders Russia, said that Moscow launched over 80 missile and heavy artillery attacks on its territory. Governor Dmytro Zhyvytsky said the strikes damaged a monastery near the border town of Shalyhyne.

Ihnat, the Ukrainian air force spokesman, said the country’s ability to shoot down incoming missiles is improving, noting there had been no recent reports of Iranian-made attack drones being used on Ukrainian territory.