Stocks drifted to a mixed close on Wall Street in what has been a rocky week because of worries coming out of the bond market about the U.S. government's mounting debt.

Bond yields inched higher and Wall Street flipped from small gains to losses before the opening bell Thursday after rising U.S. debt sank markets on the previous day.

U.S. stock index futures were muted on Friday as investors were risk-averse after President Donald Trump's tax and spending bill won a crucial House vote the previous day, raising concerns about the country's deteriorating fiscal outlook.