The State Department is giving federal agencies more time to review the Keystone XL pipeline before deciding whether to issue a permit.
That could push a decision about the controversial oil pipeline until after the midterm elections in November.
The government is not saying how much longer the review will take.
Republicans and Democrats have already accused President Barack Obama of taking too long to decide.
Last week, eleven Senate Democrats, including six who face contested races this year, urged the president to approve the oil pipeline by the end of May.
The five-year review of the Canada-to-Texas pipeline has been "exhaustive in its time, breadth and scope" and has taken longer than reasonably justified, the senators wrote to the president.
Approval of the pipeline is needed to ensure pipeline operator TransCanada does not miss
The Republican-controlled House has voted several times to approve the pipeline that would run through Cushing Oklahoma.
Polls show most Americans support the pipeline.
The pipeline also is supported by labor unions eager for the jobs it would create, as well as a range of business groups and virtually all congressional Republicans.
Environmental groups and some top Democratic donors oppose the pipeline, saying it would carry "dirty oil" that contributes to global warming. They also worry about possible spills.
The State Department has jurisdiction because the pipeline would cross the border between the U.S. and Canada.