About 400 computer experts will participate in a major cybersecurity drill in Estonia this week as part of NATO's efforts to upgrade its capability to counter potentially debilitating hacker attacks, according to AP. Teams from 16 nations will take part in the Locked Shields 2015 exercise at NATO's cyberdefense center in Tallinn. The annual drill is one of the largest of its kind. This year's drill will involve both the Windows 8 operating system and the upcoming Windows 10 system, organizers said Tuesday. The drill comes at a time of heightened tensions in Eastern Europe, where NATO military forces are exercising almost continuously to deter any Russian aggression following Moscow's intervention in Ukraine. Rob Pritchard, a cybersecurity expert at the Royal United Services Institute in London, said the cyber drill probably has a similar focus. "Russia is currently looming large on the NATO radar, and the exercise is likely to simulate attacks similar to those used by the Russian state, and state-backed actors," Pritchard said. Liisa Past, a spokeswoman for the NATO Cooperative Cyber Defence Centre of Excellence in Tallinn, said the exercise is "based on a completely fictional scenario," featuring a fictional country.