A message demanding money is seen on a monitor of a payment terminal at a branch of Ukraine's state-owned bank Oschadbank. Photograph: (Reuters)
A major ransomware attack on Tuesday hit several companies across Europe including Ukraine, France, Denmark, UK and Russia.
One of the victims of Tuesday's cyber attack, a Ukrainian media company, said its computers were blocked and it had received a demand for $300 worth of the Bitcoin crypto-currency to restore access to its files.
The attacks started in Russia and Ukraine before spreading to other European countries.
In Russia, Rosneft said a large-scale cyber attack hit its servers on Tuesday but said that its oil production had not been hit.
"The company's servers underwent a powerful hacking attack," the company said on Twitter.
"The hacking attack could lead to serious consequences, but the company has moved to a reserve production processing system and neither oil output nor refining have been stopped."
Rosneft is Russia’s top oil producer.
Russian metals giant Evraz said its IT systems had been affected too, Russia's RIA news agency reported.
Several banks and the main airport in Ukraine were also affected by cyber attacks.
The Ukrainian central bank said a number of banks and companies, including the state power distributor, were hit by a cyber attack that disrupted some operations.
"As a result of these cyber attacks these banks are having difficulties with client services and carrying out banking operations," the central bank said in a statement.
A Moscow-based cyber security firm, Group-IB, said it appeared to be a coordinated attack simultaneously targeting victims in Russia and Ukraine.
In Copenhagen, global shipping firm Shipping company AP Moller-Maersk said it had suffered a computer system outage caused by a cyber attack thought it was not immediately clear if it was connected.
Britain's WPP, the world's biggest advertising agency, also said it had been hit by a cyber attack.
Additionally, French construction materials company Saint Gobain said that it, too, had been a victim of a cyber attack, and it had isolated its computer systems in order to protect data.
"Along with other big companies, St Gobain has been the victim of a cyber attack. As a security measure and in order to protect our data, we have isolated our computer systems," said a company spokesman.