A German zoo said Tuesday its keepers had shot dead a panicked orangutan after it escaped its enclosure and threatened to run through city streets.

The adult male "Nieas" had Monday slipped through a cage door that was not properly locked, then grew agitated when confronted by a second male, said the zoo in the western city of Duisburg.

Increasingly stressed, the furry red primate swung from an overhead lamp and looked set to flee the surrounding compound from where it could have jumped a fence to escape into the city.

"Outside his familiar environment, the ape got panicked and was running back and forth scared," said the zoo in a statement.

Using tranquiliser darts was not viable because "it would have taken several minutes for the sedative to take effect", said the zoo.

"By this time the ape would already have been in city traffic and we couldn't have ruled out injury to members of the public."

The zoo had "regrettably" been forced to use live ammunition on the powerful animal, it said, adding that "the affected employees are in a state of shock".

Orangutans on the Indonesian island of Sumatra and on Borneo are threatened by deforestation and poaching.

The Sumatran orangutan is "critically endangered" while the more numerous Bornean orangutan is "endangered," according to the International Union for the Conservation of Nature.