SAN FRANCISCO/WASHINGTON - Islamic State's Twitter users, which have trumpeted the group's violent acts and worldview on the social media service, have gone abruptly quiet in past days.Several accounts affiliated with the militant group appear to have gone dormant, according to US government sources, raising questions about whether the government has pressured Twitter to clamp down more aggressively or whether the group has moved to other social media channels.When contacted, several US officials said on condition of anonymity they were unaware of attempts to quash those Twitter accounts. The sudden silence also came days after reports about Islamic State-linked accounts threatening action against Twitter employees, though there was no evidence to link the two episodes.