ROSTOV-ON-DON, July 24 (RIA Novosti) - The death toll in the south Russian road accident involving a car and a tank truck has risen to 21, a spokesman for the region's health ministry said on Friday.

A Kamaz petrol tanker crossed into the opposing lane and collided head-on with a Mercedes bus at 12:48 Moscow time [08:48 GMT] on the Don Federal Highway, some 60 km from the regional capital, Rostov-on-Don. Nineteen people were killed immediately, two later died in hospital.

"The force of the impact was so great that the cars turned into piles of scrap metal," an eyewitness told RIA Novosti. "It was impossible to determine the types of vehicles that crashed."

The local emergency said nine people were injured, while the region's health ministry says eight people were taken to the region's hospitals, all of them in a serious condition.

Russian Interior Minister Rashid Nurgaliyev, who is currently on a working visit to another south Russian region, the Krasnodar Territory, has assumed personal control of the accident investigation.

"The minister assumed personal control over the investigation into the bus accident in Rostov Region," an interior ministry spokesman said.

The minister has also instructed the head of the federal road police department, Col. Gen. Viktor Kiryanov, to visit the crash site and give an initial report on the investigation.

The network of Russian highways is one of the world's longest, but only 20% of regional roads meet international safety standards. The 1,167-km Don Federal Highway (M4), which links the capital with southern Russian regions and port cities, is considered one of the country's most dangerous.

At least 3,000 people are killed in accidents on the highway each year during the summer months, when the mostly two-lane road suffers from heavy congestion caused by summer vacationers on the way to Black Sea resorts, and by trucks transporting southern fruit to densely populated central and northern areas.