"Russia must be aggressively tested to safeguard public confidence in the integrity of the World Cup," Tygart said as quoted by The Telegraph, adding that "extraordinary performances demand additional tests."
However, FIFA refused to reveal the number of doping tests conducted on Russia's World Cup squad saying that they could not comment on such tests.
Russia qualified for the round of 16 for the first time since the break-up of the Soviet Union after defeating Saudi Arabia 5-0 in the opening game of the tournament on June 14 and beating Egypt 3-1 on Tuesday. Russian players will face Uruguay in their final group A game on Monday. Stanislav Cherchesov’s side will then play Spain, Portugal or Iran in the round of 16.
The 2018 FIFA World Cup kicked off on June 14 and will continue until July 15, with the matches being played in 11 cities across Russia.