Investigators are examining the flight box recorder of the Bashkirian Airlines' Tupolev 154 which collided with a Boeing 757 cargo plane operated by couriers DHL, late on Monday night.

But Swiss regional air traffic chief Anton Maag said both aircraft were diving to avoid a crash when they flew into each other.

And he added that the Russian pilot had started a steep dive only after controllers had repeatedly instructed him to do so.

Both planes were flying at 36,000 feet (11,500 metres) when air traffic controllers advised the Russian plane to descend.

According to Mr Maag, it took three attempts before the Russian pilot acted.

However, German crash investigators say it appears from Swiss recordings that the Russian crew reacted to their second instruction to descend.

The first instruction was given only 50 seconds before the crash, and the crew began descending about 25 seconds later, said the Federal Office for the Investigation of Air Accidents.

At the same time, the Boeing pilot was also advised to descend by the plane's collision avoidance system.

The eventual collision happened at 35,000 feet.

Earlier reports had hinted that the Russian pilot had problems understanding the air controllers, but Mr Maag said there was no language problem.

Patrick Herr, of Swiss air traffic controllers Skyguide, said there were two questions: "Why did the Russian pilot not react immediately and why did the TCAS on the 757 advise its crew to dive?"

Bashkirian Airlines has defended its pilot, saying it had been assured by the German authorities that he had not made a mistake.

Russian air ministry officials have also described the pilot and crew of the Tupolev as having enormous experience of international flying.

Axel Gietz, a spokesman for DHL, said the Boeing, with two crew on board, had been built in 1990 and had been constantly maintained.

"We cannot explain the accident at this point. We can only send our condolences to the families of those involved."

The Russian plane had been flying from Moscow to Barcelona, carrying children from Bashkortostan in the southern Urals.

Cultural visit

The children were heading to Barcelona for a festival organised by Unesco, the cultural arm of the United Nations.

Eight of the children were under 12 years old, according to Russian officials.

A three-day period of mourning has started in Bashkortostan, and President Vladimir Putin has sent his condolences.

The company said its staff were devastated by news of the tragedy, and offered its condolences to the families of the dead.

A Tupolev "black box" flight recorder has been found and will be sent for analysis.

Police boats and helicopters have been deployed on Lake Konstanz to try to find bodies and parts of the aircraft. The Boeing's flight recorder may lie on the bed of the lake.

The federal department for criminal investigations has also set up a special commission to help with the identification of the bodies - only 15 have so far been found.