Niners free safety Jimmie Ward was placed on the physically unable to perform list Friday after he sustained a strained hamstring during a conditioning test Thursday shortly after the players reported to training camp.

Ward’s timeline for recovery is week-to-week, according to a team spokesman. He can be activated from the PUP at any time to start practicing. Players who begin the regular season on the PUP can’t play for the first six weeks of the season.

“We don’t think it was a full pull or anything, but we want to be smart and safe with it,” head coach Kyle Shanahan said. “You never know with those things how long they’ll take. Don’t think it’s anything too serious, but want to definitely be safe with it.”

Ward was moved from cornerback to safety in the offseason and his position is vital in the 49ers’ 4-3 defense. Secondary coach Jeff Hafley has termed his spot “The Eraser” because Ward can clean up others’ mistakes.

On Friday, Jaquiski Tartt, a 2015 second-round pick who has focused primarily on strong safety in the offseason, assumed Ward’s position at practice. The 49ers’ other safeties are Vinnie Sunseri and undrafted rookies Lorenzo Jerome and Chanceller James.

Ward has missed 13 games in his three-year career due to injuries to his foot, quadriceps and shoulder.

Foster flashes: Rookie inside linebacker Reuben Foster made an impression in the first NFL practice in which he was a full participant.

The first-round pick intercepted a tipped pass thrown by rookie C.J. Beathard that was deflected by cornerback Prince Charles Iworah. Foster was mostly a spectator during offseason practices after undergoing shoulder surgery before the combine.

“It was good to get him out there, get him on the field,” Shanahan said. “I know he’s been chomping at the bit for a while now. It was good to see him go through it full speed, deal with getting aligned right and stuff … It was a good first day.”

Injury report: Undrafted rookie WR BJ Johnson left the field with assistance due to an injury to his left leg. He later returned to the sideline and wasn’t walking with a limp. … CB Dontae Johnson was briefly sidelined after tweaking his right shoulder.

Eric Branch is a San Francisco Chronicle staff writer. Email: Twitter: @Eric_Branch

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