President Volodymyr Zelenskyy warned Sunday that Russia is preparing to intensify its offensive along Ukraine's northern border, as the death toll rose to 14 in an aerial bomb attack on a large construction supplies store in the city of Kharkiv.

The bombing of Kharkiv on Saturday afternoon also left 43 injured and 16 missing, Kharkiv Governor Oleh Syniehubov said.

In a video statement from Kharkiv, Zelenskyy said that Russia is preparing offensive actions 90 km (55 miles) northwest of Ukraine's second largest city. He said that Russians "gather another group of troops near our border”.

He did not specify where the troops are being assembled, but Ukrainian officials have expressed strong concern about the Sumy region. Both Kharkiv city and Sumy with about 250,000 people are within about 25 km (15 miles) of the Russian border.

Moscow's troops have in recent weeks captured villages in the Kharkiv area as part of a broad push, and analysts say they may be trying to get within artillery range of the city. Ukrainian authorities have evacuated more than 11,000 people from the region since the start of the offensive on May 10.

Russian forces are carrying out offensive attacks across the 1,000-km (620-mile) front line, with pitched battles in the Chasiv Yar direction of the Donetsk region, where “the intensity of the hostilities is quite high” according to a statement from Ukraine's General Staff. “The defence forces are taking measures to prevent the advance of the enemy,” the statement said.

Russia launched wide missile and drone attacks on much of Ukraine during the night. Ukrainian officials said air defence units intercepted 31 Shahed drones and 12 cruise missiles launched by Russia in the Mykolaiv, Odesa, Dnipropetrovsk, Poltava, Zaporizhzhia, Khmelnytskyi, Kirovohrad, Vinnytsia and Chernihiv regions.

In total Russia launched 14 missiles and more than three dozen drones, according to a statement from Ukraine's air force. It was not clear what damage may have been caused by missiles and drones that were not shot down.

The governor of Russia's Belgorod region, which lies across the border from Ukraine, said four people were injured in Ukrainian attacks during the night. The Russian Defence Ministry said seven drones were shot down over the Kursk region and three over Oryol, both of which are to the north of Belgorod.