The National Bank of Ukraine said an “unknown virus” was to blame, saying several unnamed Ukrainian banks were affected along with financial firms.
“As a result of cyber attacks, these banks have difficulties with customer service and banking operations,” a statement said.
Less than 1% of the system's 9,000 users were directly impacted by the “determined and sustained” attack, officials said, but some functions were temporarily shut down as a precaution.
An increasing number of global cyber attacks, including those targeting the election campaigns of Hillary Clinton and Emmanuel Macron, have sparked warnings of a “permanent war” online.
Guillaume Poupard, director general of the National Cybersecurity Agency of France (ANSSI) said intensifying attacks were coming from unspecified states, as well as criminal and extremist groups.
“We must work collectively, not just with two or three Western countries, but on a global scale,” he added, saying attacks could aim at espionage, fraud, sabotage or destruction.
“The cyber attack could lead to serious consequences, however, due to the fact that the Company has switched to a reserve control system, neither oil production nor preparation processes were stopped,” a statement from Rosneft said.
There were confirmed reports of the virus spreading to countries including Spain, France and India.
Ukraine has blamed Russia for repeated cyber attacks targeting crucial infrastructure during the past three years, including one on its power grid that left part of western Ukraine temporarily without electricity in December 2015.
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 supplying rebels with troops and weapons.