Russian forces have blown up a gas pipeline in Kharkiv, the country's second-largest city, the Ukrainian president's office says.
Ukraine's State Service of Special Communication and Information Protection warned the explosion, which it said looked like a mushroom cloud, could cause an "environmental catastrophe".
It advised residents to cover their windows with damp cloth or gauze and drink plenty of fluids.
Ukraine's top prosecutor, Iryna Venediktova, said the Russian forces have been unable to take Kharkiv, where a fierce battle is under way.
The city of 1.5 million is located 40km from the Russian border.