By Vitalii Hnidyii

KHARKIV, Ukraine (Reuters) - Faced with the constant threat of Russian air strikes, officials in eastern Ukraine's Kharkiv region have begun building heavily fortified underground schools that will allow children to safely return to in-person studies as Moscow's invasion grinds on.

Schools in the region of around 2.5 million people, just 20 miles (32 km) from the Russian border, were forced into online learning after Russia's February 2022 invasion.

Kharkiv is frequently targeted by Russian missiles, drones and artillery, with the governor reporting on Thursday that settlements in three different districts had been struck over the previous 24 hours.

Two schools, each accommodating up to 500 people, are currently under construction and will be able to withstand direct hits, said chief regional architect Anton Korotkovskykh.

"These structures will be equipped with everything necessary for the learning process," he told Reuters in an interview.

More are planned by the end of next year, Korotkovskykh said, adding that 817 educational facilities across the Kharkiv region had been damaged or ruined during Russia's invasion.

On a recent morning, workers welded together layers of steel reinforcing wire and poured cement, occasionally to the sound of the air-raid sirens that regularly ring out.

The spaces will be equipped with fully functional ventilation, plumbing and heating systems, said Serhiy Petrulyanis, whose construction firm is building one of the schools.

"That is, people will be able to remain here more than just one day," he said.

Ukrainians are now bracing for another winter of likely Russian air strikes targeting critical infrastructure, which last year caused widespread blackouts across the country.

(Reporting by Vitalii Hnidyi; Writing by Dan Peleschuk; Editing by Gareth Jones)