MOSCOW (Reuters) – As many as 40 people could still be trapped in the rubble of a Russian apartment block that partially collapsed in an explosion on Monday, killing at least four people, news agencies reported.

The blast, thought to have been caused by a gas leak, damaged 48 apartments in a nine-story building in Magnitogorsk, an industrial city in the Urals some 1,700 km (1,050 miles) east of Moscow, the emergencies ministry said.

Emergency personnel work at the site of collapsed apartment building after a suspected gas blast in Magnitogorsk, Russia December 31, 2018. Minister of Civil Defence, Emergencies and Disaster Relief/Handout via REUTERS.

President Vladimir Putin flew into Magnitogorsk late on Monday afternoon, visiting the injured in hospital and meeting with local authorities, state television showed.

Putin looked on as rescue workers toiled in temperatures of -22 Celsius (-8 Fahrenheit) to locate people trapped in the debris.

Emergencies Minister Yevgeny Zinichev said at a meeting with Putin there were “presumably between 36 and 40 people under the rubble,” agencies reported.

The ministry told Russian agencies earlier on Monday that five people were in hospital and that the fate of 35 people was unknown.

Health Minister Veronika Skvortsova told state television that the chances of finding survivors were diminishing as the day wore on.

The blast tore through the building at around 6 a.m. (0100 GMT) when many residents were asleep, RIA news agency reported. Monday was a public holiday in Russia.

There have been several similar incidents in Russia in recent years due to aging infrastructure and poor safety regulations surrounding gas usage.

In 2015, at least five people were killed when a gas explosion damaged an apartment building in the southern city of Volgograd.

(Reporting by Vladimir Soldatkin and Gabrielle T’trault-Farber; editing by Richard Balmforth and Robin Pomeroy)