A police spokeswoman said the passengers were "old people" and that some may not have made it out of the bus.
18 feared dead in Germany bus accident
Up to 18 people were feared dead in one of Germany's worst road accidents after a tour bus carrying pensioners burst into flames after colliding with a lorry, police said.
"Thirty passengers were taken to hospitals, some with serious injuries. The others are believed to have died in the burning tour bus," which had been carrying 48 people, police said.
There were 46 passengers and two drivers in the bus when it rammed into the trailer in a traffic jam on the motorway A9 close to the Bavarian town of Stammbach.
The crash took place shortly after 7am and sent the bus up in flames, with television images showing only the charred skeleton of the vehicle remaining.
"Unfortunately numerous people - a tour group from [the eastern German state of] Saxony - lost their lives in this burning bus," government spokesman Steffen Seibert told a news conference. Some of the 30 passengers pulled out of the burning bus were seriously hurt, he said.
Another police spokeswoman, Anne Hoefer, said the passengers were "old people" and that some may not have made it out of the bus.
Chancellor Angela Merkel voiced great dismay over the crash, with her spokesman saying: "Our thoughts and condolences go to the victims and their family members, as well as to the injured. We hope that those who have been rescued will recover from their injuries."
Transport minister Alexander Dobrindt and Bavaria state's interior minister Joachim Herrmann visited the site of the accident.
According to the Bild newspaper, the coach was travelling from the eastern region of Lausitz and heading for Nuremberg.
The accident struck near the town Bayreuth, which draws thousands of classical music lovers to its opera festival every summer.
The region close to the Czech border is also dotted with spas and castles, and the A9 is a trunk motorway leading to many popular summer holiday spots.
Some 200 emergency workers were deployed to the site, including firefighters, rescue workers and police, while south-bound traffic on the motorway remained blocked.
Those hurt were being flown by rescue helicopters to hospitals, many with life-threatening injuries.
With the likely toll, the accident at the start of the summer holiday season is one of the worst to hit Germany.
Among the deadliest in recent years was a collision in June 2007, when 13 people were killed as their tour bus drove off the road and plunged several metres down a slope in eastern Germany's Saxony-Anhalt state.
In September 1992, 21 people died when a bus swerved out of its lane and struck a truck before ramming into the road divider in the southern Black Forest region, a key tourist destination.
Across Europe, the last such fatal accident struck was on January 21 in Italy, when an accident involving a Hungarian coach carrying 56 people burst into flames after crashing on a highway near the northern city of Verona. Sixteen people died, most of them teenagers returning from a ski trip to France.
In France, at least 43 people were killed in October 2015 when a coach carrying members of a pensioner's club collided with a lorry and burst into flames near Puisseguin in southwest France. It was the country's worst road accident in three decades.
* Agence France-Presse and Reuters