More than 1000 cranes are thought to have died in a large outbreak of bird flu near Berlin, German conservationists recovering the animals' bodies say.

A total of 839 dead cranes have been counted since Sunday in a pond area in Linum, about 30km northwest of the capital, the head of the Rhinluch Species Protection Centre, Norbert Schneeweiß, said.

Altogether about 1500 infected animals may have died, according to Schneeweiß, who is co-ordinating a team of helpers searching deeper and more inaccessible waters.

"We are still surrounded by dying animals," said Schneeweiß, who has been organising crane conservation efforts for years.

The helpers, who have been working for days, are "fully equipped from head to toe" and must disinfect themselves after dealing with the birds.

"It is dramatic for the cranes. But it is also taking a toll on the helpers," he added.

"This is now really a case of mass mortality. We need professional support," he previously told the Radio Eins broadcaster: "This is not something that nature lovers can do in their spare time."

The group called upon the fire brigade to lend assistance while local authorities hoped for technical support at the federal level.

A drone was also flown over the area to assess the situation.

Every year, tens of thousands of cranes stop in the state of Brandenburg, which surrounds Berlin, on their way to their southern wintering grounds.

The conservationists fear that other species may also be affected.

Ravens and birds of prey that feed on carrion are active in the area.

Several states in eastern Germany reported high crane mortality over recent days amid outbreaks of bird flu, the federal authority responsible for monitoring the disease said.

The Friedrich Loeffler Institute (FLI) confirmed infection with an avian influenza virus known as HPAIV of the subtype H5N1, generally known as bird flu, in crane samples from four states.

In its current risk analysis, the FLI raised its risk assessment from "low" to "high" for poultry farms and from "low" to "moderate" for wildfowl.

While bird flu is a highly infectious disease with a high mortality rate among poultry and wildfowl, it is harmless to humans.

The institute recommends rapid removal of carcasses to prevent further spread of the virus.

People, including poultry farmers, should avoid contact as much as possible.