BRUSSELS (Reuters) - The European Union is seeing signs on social media that Russia could introduce martial law in the country after its invasion of Ukraine, an EU official said on Thursday. The official said the bloc was picking up speculation on social media about potential Russian plans, which it said would be "completely home-produced". "As is the tragic loss of young lives killed in the military conflict, with Russian mothers having to learn about the loss of their sons. So it is something we're conscious of. And it's something we're worried about," the official said. (Reporting by John C...