KYIV—Kyiv believes a hacker group linked to Belarusian intelligence carried out a cyberattack that hit Ukrainian government websites this week and used malware similar to that used by a group tied to Russian intelligence, a senior Ukrainian security official said. Serhiy Demedyuk, deputy secretary of the national security and defense council, told Reuters that Ukraine blamed Friday’s attack—which defaced government websites with threatening messages—on a group known as UNC1151 and that it was cover for more destructive actions behind the scenes. “We believe preliminarily that the group UNC1151 may be involved in this attack,” he said. His comments offer the first detailed analysis by Kyiv on the suspected culprits behind the cyberattack on dozens of websites. Officials on Friday said Russia was probably involved but gave no details. Belarus is a close ally of Russia. The cyberattack splashed websites with a warning to “be afraid and expect the worst” at …