9 February 2018 12:15 pm
Lawmakers had hoped to approve a new spending bill before federal funding expired at midnight.
However, Republican Senator Rand Paul has ended hopes for a quick vote when he demanded a debate in the chamber on his amendment to maintain spending caps.
In January, a similar failure to pass a bill led to a three-day government shutdown.
The federal Office of Personnel Management said government operations would "vary by agency".
"Employees should refer to their home agency for guidance on reporting for duty," it said in a brief statement.
Both the Senate and the House of Representatives need to approve the new two-year spending deal.
The shutdown was essentially guaranteed an hour before the deadline, when the Senate voted for a recess until 00:01.
Despite the delays, the Senate is due to vote on the budgetary measure after 01:00. The House will not vote on the deal until the Senate approves it.