A Nebraska court in February ruled legislation that gave Nebraska Gov. Dave Heineman authority over the pipeline's route violated the state's constitution.
U.S. State Department spokeswoman Jen Psaki told reporters during her regular press briefing that, while it's largely a state matter, it was best to wait on deciding whether or not to issue the necessary federal permit to build the cross-border pipeline.
"We felt it was the appropriate step to extend the timeline," she said Monday.
Pipeline company TransCanada submitted an application to build the cross-border pipeline more than five years ago. TransCanada Chief Executive Officer Russ Girling said a notice of appeal staying a lower Nebraska's court decision means Heineman's approval remains in effect. Further delays, Girling said, were "inexplicable."
"Canadian oil will make its way to market with or without Keystone XL," he said in a statement Monday. "It is in everyone's best interests that this project move forward."
Supporters of Keystone XL say it will add a layer of security to the North American energy sector and provide a source of economic stimulus. Opponents say the more viscous form of Canadian crude oil it's designed to carry is a grave environmental threat.