Hertha Berlin's goalkeeping coach has urged U.S. youth international Jonathan Klinsmann to work harder, saying there's "too much American" in the 20-year-old.

Klinsmann, the son of former Germany star and U.S. coach Jurgen Klinsmann, has made nine appearances for Hertha's under-23 team in the German fourth division and one for the senior team, but he's behind former Bayern Munich goalkeeper Thomas Kraft, 29, and Norway international Rune Jarstein, 33, in the club's pecking order.

And speaking to reporters on Monday, Hertha assistant Zsolt Petry issued a warning to the younger Klinsmann, saying he must improve his work ethic.

"He's got the basic skills, and taken a good athletic development," Petry said. "But he does not bring his skills to the pitch every day. All of the coaching staff misses that.

"The past two, three months did not go as we hoped. The serious, focused German working is not fully in him, there's too much American left in him.

Jonathan Klinsmann has made one first-team appearance since signing for Hertha Berlin in July. Alex Livesey/FIFA via Getty Images

"His personality development has stalled a bit. You can see that in training. He does not talk too much to his teammates, has little contact, not a lot of charisma, confidence and determination.

"One word: The body language is still missing. Without it you won't get far in football. If he can do it he can become an option."

Klinsmann went on trial at several European clubs after playing in college at California and with the U.S. at the Under-20 World Cup, before signing for Bundesliga side Hertha last July.

Klinsmann was born in Germany but said earlier this week that he is still learning the language after growing up in the U.S. However, he said reconnecting with his German heritage was among his biggest goals.

"I didn't want to go to England or America because I knew English already," Klinsmann said. "For me, German was unfinished and I wanted to be able to integrate myself in Germany.

"I was born here. I was born in Munich. I wanted to be able to come here and speak freely. I wanted to learn German. That was definitely one of my top priorities."