Dispatchers inside the control room of Ukraine’s National power company in Kiev.(Reuters File Photo)

A number of Ukrainian banks and companies, including the state power distributor, were hit by a cyber attack on Tuesday that disrupted some operations, the Ukrainian central bank said.

The latest disruptions follow a spate of hacking attempts on state websites in late-2016 and repeated attacks on Ukraine’s power grid that prompted security chiefs to call for improved cyber defences.

The central bank said an “unknown virus” was to blame for the latest attacks, but did not give further details or say which banks and firms had been affected.

“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.

“The central bank is confident that the banking infrastructure’s defence against cyber fraud is properly set up and attempted cyber attacks on banks’ IT systems will be neutralised,” it said.

Oschadbank, one of Ukraine’s largest state-owned lenders, said some of its services had been affected by a “hacking attack” but guaranteed that customer data was safe.

Ukrainian state power distributor Ukrenergo said its IT system had been hit by a cyber attack, but the disruption had no impact on power supplies or its broader operations.

In Russia, its top oil producer Rosneft said a large-scale cyber attack had hit its servers.

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 A.P. 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.

One of the firms Group-IB said had been hit in Russia, Damco, which is Maersk’s logistics company.

Rosneft, one of the world’s biggest producers of crude oil by volume, said its oil production had not been affected.

“The company’s servers underwent a powerful hacking attack,” the company said on Twitter

Russian metals giant Evraz said its IT systems had been affected too, Russia’s RIA news agency reported.

Ukraine has blamed Russia for previous cyber attacks, including one on its power grid at the end of 2015 that left part of western Ukraine temporarily without electricity.

Cyber security experts consider Russia one of the world’s most advanced cyber powers, along with the United States, China, Israel, France and Britain.

Relations between Kiev and the Kremlin collapsed in 2014 following Moscow’s annexation of Crimea and support for pro-Russian separatists in eastern Ukraine, where fighting continues despite a ceasefire agreement.

Russia denies carrying out cyber attacks on Ukraine and allegations it has fuelled the eastern conflict by suppling rebels with troops and weapons.