Seven Iranians have been indicted by a U.S. federal grand jury on charges related to cyber attacks on infrastructure and banks, the Justice Department said Thursday.

"Over the course of 176 days, servers of 46 financial institutions were intruded," Attorney General Loretta Lynch said of the attacks from 2011 to 2013.

The defendants were identified as Ahmad Fathi, Hamid Firoozi, Amin Shokohi, Sadegh Ahmadzadegan, Omid Ghaffarinia, Sina Keissar and Nader Saedi.

The "experienced hackers" were employed by Iran-based computer security companies, ITSecTeam and Mersad Company, that worked on behalf of the Iranian government and the Islamic Revolutionary Guard Corps, according to Lynch, who said the attacks were "a large scale and coordinated campaign of distributed denial of service, or DDOS".

DDOS attacks on banking online services disrupt accounts and block users from accessing their accounts.

Firoozi was charged with "illegally obtaining the data to access supervisory control and data acquisition system" of a dam in New York state.

At the time of the alleged intrusion, the dam was not connected to the computer system because it was under maintenance, Lynch said.

If the dam were connected, she said, the breach would have been "a clear and present danger to the public health and safety of Americans".

FBI Director James Comey, speaking alongside Lynch, said the hackers are in Iran but the bureau is "prepared to nab them" if they travel outside of the country.

The government did not disclose how it made the connection between the hackers, companies and the Iranian government.

The indictment comes less than a year after a nuclear deal was signed between Iran, the U.S. and world powers. In recent years, the U.S. government and private sector computer systems have suffered several major attacks by Russian, Chinese and North Korean hackers.

Last year Chinese hacker allegedly breached federal government database stealing information of millions of federal government employees.