Russian space agency Roscosmos said the situation is under control and the crew – three Russians, two Americans and an Italian – were safe in the Russian section.
"The US section of the ISS has been isolated, the crew are safe and in the Russian section," Maxim Matyushin, head of Russia's Mission Control Centre, said in a statement.
"The concentration of impurities in the atmosphere in the Russian section of the ISS is within permissible levels."
Mr Matyushin said it was now up to the US to decide what further action might have to be taken to deal with the problem.
The crew's health was not in danger, it added.
The space station has previously suffered ammonia leaks.
Since 2009 there have been teams of six astronauts and cosmonauts aboard the ISS.