Funding for key government operations expired at 11:59:59 p.m. on Friday (around 5:00 a.m. GMT on Saturday). Earlier in the day, the Senate failed to reach a compromise over a House-passed bill and decided to adjourn until Saturday.
The federal government previously closed twice this year with minimal impact. A three-day shutdown in January took place over a weekend and a February closure only lasted for a day. All three shutdowns this year involved disputes over immigration.
However, Donald Trump expressed hope on Friday that the shutdown will not last long.
On Wednesday, the US House of Representatives passed the spending bill allocating $5 billion for the construction of the wall on the border with Mexico. The Senate lawmakers, who have opposed the border wall project, however, adjourned on Friday, failing to avert the shutdown.