Britain's WPP, the world's biggest advertising company, said on Tuesday that computer systems within several of its agencies had been hit by a suspected cyber attack.
Several major companies, including Russia's top oil producer Rosneft, said they have also been affected by a large scale cyber attack.
"IT systems in several WPP companies have been affected by a suspected cyber attack," the company said. "We are assessing the situation, taking appropriate measures and will update as soon as possible."
WPP owns agencies including JWT, Ogilvy & Mather, Young & Rubicam and Grey.
Russia's top oil producer Rosneft said a large-scale cyber attack had hit its servers and computer systems at some banks and the main airport in neighbouring Ukraine were also disrupted.
