Washington (AFP) - Hackers demanding ransom payable in bitcoin have attacked computers of the Atlanta city government in the southern US state of Georgia, officials say.

The ransomware assault shut down multiple internal and external applications for the city, including apps that people use to pay bills and access court-related information, Mayor Keisha Lance Bottoms told a news conference Thursday.

"This is a very serious situation," Bottoms said.

City officials said they learned of the attack before dawn Thursday when they detected unusual activity on their servers. Some of the city's data had become encrypted.

A ransom note sent to the city gave instructions for paying to free up files encrypted by the hackers.

Investigators say the total ransom being demanded was around $51,000 payable in bitcoin, local news media reported.

The FBI and the Department of Homeland Security were called in to help investigate.

Last year a ransomware attack hit more than 200,000 companies, hospitals, government agencies and other organizations in 150 countries.

Those attackers were believed to have used tools developed by the US National Security Agency and leaked by a hacker group that calls itself the Shadow Brokers.