Watch video: Russian forces blow up gas pipeline in Ukraine's Kharkiv

The Ukrainian president's office said Russian forces blew up a gas pipeline in Kharkiv, the country's second-largest city.

The State Service of Special Communication and Information Protection warned that the explosion, which it said looked like a mushroom cloud, could cause an "environmental catastrophe" and advised residents to cover their windows with damp cloth or gauze and to 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 underway.

The city of 1.5 million is located 40 kilometres (25 miles) from the Russian border.