Thousands of computer users across the globe scrambled to reboot on Wednesday after a wave of ransomware cyber-attacks spread from Ukraine and Russia through Europe to the United States.
The virus, which demanded a payment worth $300 (roughly Rs. 20,000) as it locked up files at companies and government agencies including the Chernobyl nuclear site, appeared similar to the WannaCry ransomware that swept the world last month, hitting more than 200,000 users in more than 150 countries.
But the new attack appeared much smaller in scale, with global cyber-security firm Kaspersky Lab estimating the number of victims at 2,000. There was no immediate indication of who was responsible.
Some IT specialists identified the newcomer as "Petrwrap", a modified version of ransomware called Petya which circulated last year. But Kaspersky described it as a new form of ransomware.
The government of Ukraine, where the attacks were first reported and appeared most severe, said the attack had been halted, but key organisations were still reporting problems.
"The large-scale cyber-attack on corporate and government networks that happened yesterday on June 27 has been stopped," the government said in a statement.
"The situation is under the complete control of cyber-security experts and they are now working on recovering lost data," it said, adding that all "strategic enterprises" were functioning as normal.
Despite the assurances, employees at the Chernobyl nuclear site were continuing to use hand-held Geiger counters to measure radiation levels after the monitoring system was shut down by the hack.
Online arrivals and departures information for Kiev's main Boryspil airport also remained down, but its director said the hub was otherwise fully operational.
Meanwhile, systems at the major lender Oschadbank still appeared crippled, while a delivery service and energy supplier said they were also facing some difficulties.
The attacks started Tuesday at around 2:00pm in Kiev (1100 GMT) and quickly spread to about 80 companies in Ukraine and Russia, said cyber-security company Group IB.
In Russia, major companies including the oil giant Rosneft said they had suffered cyberattacks at roughly the same time.
Later, multinationals in Western Europe and the United States reported that they too had been hit by the virus.
Among the companies reporting problems were global shipping firm Maersk, British advertising giant WPP, French industrial group Saint-Gobain and US pharmaceutical group Merck.
India's government on Wednesday said operations at a terminal at the country's largest container port in Mumbai, run by Maersk, were disrupted.
Security specialists said the cyber-attacks on Tuesday exploited an already patched vulnerability in Windows software and appeared to have focused on Ukraine as a primary target.
The malware that, once in a computer, locked away data from users who were then told to pay, bore resemblances to the recent WannaCry attack. US software titan Microsoft also called the latest virus ransomware.
"Our initial analysis found that the ransomware uses multiple techniques to spread, including one which was addressed by a security update previously provided for all platforms from Windows XP to Windows 10 (MS17-010)," a Microsoft spokesperson told AFP.
After the WannaCry scourge in May, Microsoft urged users to protect machines with the MS17-010 patch.
The flaw - and the means to exploit it - had previously been disclosed in pirated documents about cyberweapons at the US National Security Agency.
So far there was no clear indication of who was behind the attack.
Some experts said it looked likely to be a criminal scam, while Ukraine suggested that its archrival Russia could have been behind the attack.