Speaking after Wednesday's 4-1 loss in a friendly to Brazil, in which brilliant youngster Neymar scored a goal and had a hand in two others, Klinsmann said it was time the U.S. adopted a more hard-line approach.
"I think we've got to get an edge, more nastier," he told a news conference.
"Maybe we're still a little bit too naïve, maybe we don't want to hurt people, but that's what you've got to do.
"We've got to step on their toes more and get them more frustrated, and make a case with the referee as well, for us, not only the opponents."
The Americans had won their last five internationals in a row, including a 5-1 thumping of Scotland last weekend, but were brought back down to earth by a youthful Brazilian team preparing for the London Olympics.
Neymar opened the scoring after just 12 minutes when he coolly slotted home a penalty after U.S. defender Oguchi Oneywu accidently handballed in the box.
The 20-year-old set up the second goal for the South Americans when he laid on a pinpoint corner that defender Thiago Silva headed past U.S. goalkeeper Tim Howard and into the net.
The Americans, who are in the middle of a hectic schedule of warm-ups for the 2014 World Cup qualifiers, pulled one back on the stroke of halftime when Herculez Gomez, starting an international for the first time since the 2010 World Cup, nodded in a cross from Fabian Johnson.
Klinsmann was impressed by Gomez, who was starting an international for the first time since the 2010 World Cup, saying he had shown the right approach.
"I think we've found a striker that is very mobile, in Herculez, one who is a fighter, who keeps defenders busy, who is also nasty to do certain things," Klinsmann said.
"That's important and good to see, that gives more movement up front there. We're not scoring enough goals yet so we're going to give that message to our strikers to go for it, and Herculez did a good job."
The Americans created a handful of chances in the second half but were denied by some acrobatic saves from Brazilian goalkeeper Rafael, who impressed on debut, and some old fashioned bad luck when Onyewu hit the bar.
Brazil were far more clinical when they had opportunities. Neymar cut the ball back inside to present Marcelo with an easy goal after 59 minutes then substitute Alexandre Pato completed the rout when he angled the ball in to score three minutes from time.
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