Iranian government has pledged inquiry into her death.
The warning was a clear indication that the elite force is prepared to step up its crackdown on demonstrations.
Protesters in Tehran and other Iranian cities torched police stations and vehicles earlier on Thursday as public outrage over the death showed no signs of abating, with reports of security forces coming under attack.
The United States on Thursday imposed sanctions on Iran's morality police, accusing them of abuse and violence against Iranian women and of violating the rights of peaceful Iranian protesters, the U.S. Treasury said.
On Twitter, WhatsApp said it is working to keep Iranian users connected, adding that it is not blocking Iranian numbers.
In the northeast, protesters shouted "We will die, we will die but we'll get Iran back" near a police station which was set on fire, a video posted on Twitter account 1500tasvir showed.
Iranian officials have denied that security forces have killed protesters, suggesting they may have been shot by armed dissidents. »