Atlanta's officials battled to restore critical systems over the weekend after a nine-day-old cyber attack crippled the American city's computer network, forcing workers to resort to paper.

Government files, including police and financial documents, were made inaccessible after hackers released a "ransomware" virus on the network, demanding US$51,000 (S$67,000) worth of bitcoin.

The city's mayor declined to say if a ransom was paid, after a local television station released an image of an extortion note.

SEE TOP OF THE NEWS: US officials struggle to restore systems after Atlanta cyber attack