LONDON (Reuters) - World stocks extended a steep sell-off on Friday as the threat of a U.S. government shutdown and further hikes in U.S. borrowing costs compounded investor anxieties over the trajectory of global economic growth. A man walks past in front of a stock quotation board showing the price of the SoftBank Corp. and Nikkei share average outside a brokerage in Tokyo, Japan December 19, 2018. REUTERS/Issei Kato European shares opened in negative territory, following in the footsteps of U.S. and Asian markets.