Poland scrambled fighter jets to protect its airspace Tuesday as Russia intensified its bombardment of Ukraine with missiles. Last week one Russian missile crossed briefly into Polish territory, according to the Polish military.

Poland, a NATO member that shares a border with Ukraine, said it activated two pairs of F-16 fighter jets and an allied air tanker early Tuesday morning, after Russia launched its latest deadly missile attack on Ukraine, targeting the capital Kyiv and eastern Kharkiv region.

“We would like to inform you that intensive long-range aviation activity of the Russian Federation is being observed, which is related to carrying out strikes on the territory of Ukraine,” Poland’s Operational Command wrote on X. It said it activated a pair of fighter jets stationed at a base in Łask and in Krzesiny “in order to ensure the safety of Polish airspace.”

The decision comes days after the Polish military said it believed a Russian missile had entered and then left Polish airspace. Russia had stepped up its strikes on Ukraine over the New Year period and on Friday unleashed the biggest missile attack since the start of its full-scale invasion nearly two years ago.

Polish Deputy Foreign Minister Władysław Teofil Bartoszewski summoned Russia’s chargé d’affaires Andrei Ordash and gave him a note requesting an explanation. But Russia said it “will not give explanations” until concrete evidence is presented, Russian state media RIA Novosti reported.

Ordash told RIA Novosti that the note “contained only unfounded accusations,” adding that Poland had “refused to provide evidence that the missile was of Russian origin.”

Russia mounted another heavy missile and drone attack on Ukraine overnight into Tuesday, striking residential buildings and civilian infrastructure across the country, after the Kremlin pledged that Ukraine’s attack on the Russian city of Belgorod “will not go unpunished.”

At least 25 people were killed and 108 others injured in the Ukrainian attack Saturday, Russian authorities said.

Russia’s Defense Ministry said it would avenge the “crime” and its military has bombarded Ukraine with missiles every day since.

At least four people were killed and 92 injured in the attacks Tuesday morning, according to Ukrainian President Volodymyr Zelensky.

“Our air defense soldiers have been doing an incredible job for three days now. Since December 31, the Russian inhumans have already used about 170 ‘Shaheds’ [drones] and dozens of missiles of various types. The vast majority of them were aimed at civilian objects,” Zelensky wrote on Telegram.

‘City is in flames’

Russia’s overnight strikes had caused seven fires in six districts of Kyiv, Ukraine’s Interior Minister Ihor Klymenko said Tuesday. More than 130 people were evacuated from a high-rise building in Solomainskyi district, where 20 people were rescued. At least 20 people total were injured in the capital.

In the Kyiv region, two people were killed in a fire at a residential building in the Fastiv district. In the city of Vyshneve at least seven people were injured, and five apartment buildings and 40 cars were damaged.

Peter Zalmayev, Director of the Eurasia Democracy Initiative, said he woke in Kyiv Tuesday morning to find “the entire city is in flames.”

He said some energy infrastructure also seemed to have been struck, in what he called a reminder of Russian President Vladimir Putin’s attempt last year to break Ukraine’s power grid and weaponize the winter weather.

“It may be that the start of 2024 is the start of a renewed campaign to try to knock out our power facilities and our water supplies,” Zalmayev said.

Tension at the border

Poland’s border with Ukraine runs for more than 300 miles (500 kilometers) and has been the site of numerous scares since the start of Russia’s full-scale invasion, as its war on Ukraine threatens to spill over into neighboring European countries.

In November 2022, a Russian-made missile landed outside the rural Polish village of Przewodow, about 4 miles (6.4 kilometers) west of the Ukrainian border. Two Polish citizens were killed by the blast, which marked the first time a NATO member had been directly hit during the conflict.

Both Russian and Ukrainian forces have used Russian-made munitions during the conflict, with Ukraine deploying Russian-made S-300 missiles as part of its air defense system before being supplied with the superior US-made Patriot system.

Despite initial fears that Russia may have struck Poland in 2022, Polish President Andrzej Duda said at the time there was “no indication that this was an intentional attack on Poland.” NATO Secretary-General Jens Stoltenberg concurred with the assessment.

Instead, Duda said there was a “high chance” it was an air defense missile from the Ukrainian side that had fallen in Poland in “an accident” while intercepting incoming Russian missiles. He confirmed the rocket was most likely a Russian-made S-300.

During Friday’s air attack – the largest on Ukraine since the start of Russia’s full-scale invasion – Poland’s military reported an “unidentified airborne object” entered Polish airspace from Ukrainian territory.

Poland’s most senior military officer General Wiesław Kukuła said “all indications” suggested the object was a Russian missile. NATO Secretary-General Jens Stoltenberg said Friday that the alliance “remains vigilant” after the incident.

This is a developing story and will be updated.