1.4 million civilians in Kharkiv, Ukraine's second-largest city, have been told to take shelter as Russian troops begin siege

• Russian forces invaded Ukraine's second-largest city, Kharkiv, overnight.
• The city's governor urged its 1.4 million residents to remain indoors.

The 1.4 million residents in Ukraine's second-largest city, Kharkiv, have been urged to shelter indoors after government officials said Russian troops had arrived.

In a statement on Sunday morning, Kharkiv's governor Oleg Sinegubov told people not to leave their homes after a "breakthrough of light equipment of the Russian enemy" reached the central part of the city.

A video verified by Sky News shows tanks driving into the city, which is located just 20 miles south of the Russian border.

Russian troops blew up a nine-story residential block overnight, killing one elderly woman, the BBC reported, citing emergency services.

The building was severely damaged but rescuers said around 60 residents were spared injury after they had taken refuge in the basement, the BBC said.

Russian forces also blew up a major gas pipeline in the city, which the Ukrainian government said could result in an "environmental catastrophe," the Associated Press reported. Residents were urged to cover their windows with damp cloth or gauze, per AP.

A group of scientists from the Nationwide Academy of Sciences of Ukraine sheltering in a Kharkiv on Saturday night, said that the situation on the ground is looking dire.

They wrote in an email that a kindergarten and a hospital had been bombed and children killed. "There are many saboteurs who paint fluorescent labels for missiles on residential buildings. They are on the roofs of houses, on the gas pipes of ordinary high-rise buildings. This is an absolute horror and a reality that is hard to believe even today," they said.

On Sunday afternoon, Kharkiv's regional governor Oleh Sinegubov claimed that Ukrainian forces had taken "complete control" over the city after clashing with Russian forces.

"Control over Kharkiv is completely ours. The armed forces, the police and the defense forces are working, and the city is being completely cleansed of the enemy," he wrote on Telegram.

The attacks come on the fourth day of President Vladimir Putin's all-out invasion of the country.

In a brief televised address this morning — marking his first public remarks since Friday — Putin saluted the "heroism" of Russian special forces fighting in Ukraine, the Guardian reported.

He referred to the invasion as a "special operation to provide assistance to the people's republics of the Donbas," per the Guardian.

Insider's live blog of the invasion is covering developments as they happen.