The Federal Road Traffic Inspection agency said the crash happened at 5:45 am local time (0245 GMT) in the Ryazan region.

"As a result of the accident five people died. Twenty-one were injured," the agency said on Telegram, adding that two of the injured were under-age.

It said the injuries were of "various severity".

The agency published photographs of the badly damaged coach, which hit a pillar under a railway bridge. The front of the bus appeared to be entirely crushed.

The driver was among the fatalities, and an internet website belonging to the Russian Interior Ministry said he could have fallen asleep behind the wheel.

The accident took place on a highway near the village of Voslebovo 270 kilometres (170 miles) south of the capital, the agency said.

The bus carried a total of 49 passengers, travelling from the southern city of Astrakhan on the Volga River to Moscow.

Police said they had opened a criminal case for breaking traffic rules.

The local branch of Russia's Investigative Committee -- which probes major crimes -- later said it had opened a second criminal case into violations of safety rules.

Violations of travel safety requirements are common in Russia, which has seen a number of deadly bus accidents in recent years.

In December 2019, a bus plunged off a bridge into a frozen river in Siberia, killing 19 people.