Analysts fear the political deadlock in Washington could risk the US defaulting on its debt for the first time, sending prices soaring and global stock markets into turmoil.
Congress has until 17 October to agree on an increase to the $16.7 trillion borrowing cap, but Republicans and Democrats appear no closer to a resolution.
The last time the US came close to a default in 2011, stocks in America's largest companies fell some 17 points and one credit rating agency removed the country's top-tier rating.
The episode cost the US Treasury $1.3 billion (£803 million) in higher interest costs that year, according to a government study.
The US shutdown is but a precursor to a much bigger problem - in a couple of weeks the government, shutdown or not, will run out of cash.
Landmarks across the US have closed as the Congress failed to reach a deal over the budget.