RAMAT GAN, Israel, May 6 (Reuters) - A five-year-old hippopotamus named Lieber died in an Israeli safari park on Wednesday as he was being castrated by an international medical team.

A German veterinarian jumped up and down on the unconscious animal's 1.5-tonne body in an unsuccessful effort to restart Lieber's heart, which had stopped after a second dose of anaesthetic was administered to repair a torn suture.

Sagit Horowitz, a spokeswoman for the Ramat Gan Safari Park near Tel Aviv, said its hippo herd of 40 was the largest in captivity in the world and officials feared any further population growth could lead to aggression among the animals.

Four other males at the park survived their castration and another was scheduled to undergo the procedure later in the day, she said.

"The risk involved is very high, we knew (this) before we started," said Austrian vet Chris Walzer who came to Israel along with two colleagues, a German and Frenchman, to castrate the hippos, whose testicles are internal. (Additional reporting by Rami Amichai, writing by Ori Lewis in Jerusalem; Editing by Jon Hemming)