Kyle Shanahan praised the aggression of his San Francisco 49ers, but warned they must channel it appropriately after stopping practice earlier in the week due to fighting.

Tuesday's practice was reportedly halted by a scuffle between linebacker Fred Warner and receiver Brandon Aiyuk, with punches thrown.

Speaking to reporters on Friday, Shanahan said he wanted to see his players push one another, metaphorically, but that it goes too far when they could potentially cause each other injuries.

"I want everyone challenging each other," Shanahan said. "I don't care how much crap each other talks; I don't care how close they get to fighting.

"They can do whatever they want to get themselves to be as intense as they want and bring the best out of each other, which happens a lot that way. And it's the same on the field.

"But once you throw a punch, you get ejected or you get a penalty... I want people to be irritants, I want people to get as close as they can to all that stuff. I want people to get right on the line where they're about to black out, but you can't black out on the football field or you cost your team."

The disagreement between Warner and Aiyuk reportedly erupted after the former hit receiver Marcus Johnson late, which put Johnson in concussion protocol.

"I love the intensity of it," Shanahan added. "I don't think you have to fight to be intense, though. Scuffles are scuffles but then they lead to other stuff.

"I think that's why we've got a guy in protocol, because he took an unnecessary shot on someone, which led to the big fight and then we had a bunch of haymakers and stuff thrown in there, which only break hands.

"I think our team is pretty tough. I think we're pretty physical. Most probably [we would be] voted the most physical team on tape last year, I think we'd win most of that, and we didn't get in one fight last year. So, I don't think that totally pertains to toughness."