While President Donald Trump works to advance two controversial oil pipelines – Dakota Access and Keystone XL, a war's being fought in Washington over a whole other set of lines: natural gas ones.

Take Rover, a gas pipeline proposed by Energy Transfer Partners, the same company building Dakota Access. It's been waiting for approval from the Federal Energy Regulatory Commission for two years. In fact, as Trump was declaring his support for the oil lines on Tuesday, a would-be shipper on Rover was asking the regulatory commission to expedite its decision on the gas project.

Trump has pledged to accelerate approvals for energy infrastructure, but lines like Rover underscore the unique challenges facing the gas industry. The Federal Energy Regulatory Commission, charged with reviewing gas lines, was established as an independent agency when it was formed by Congress in 1977. As environmental opposition to gas projects mounts, the time it takes for the commission to review them has also risen – to average 429 days by Bloomberg Intelligence's estimates.

Dakota Access and Keystone are "the most politicised infrastructure projects out there," so it makes sense for Trump to address them first and "set the tone for the early days of his administration," said Brandon Barnes, an energy analyst at Bloomberg Intelligence. "There might be a delay in the gas pipes feeling the same kind of love."

Regulatory battles

At least five major gas pipelines planned across the US. are caught up in legal battles and lengthy reviews before energy regulators. The lack of pipeline space is meanwhile depressing prices for gas flowing out of prolific shale formations in the eastern US., forcing drillers to curb output.

Energy explorer Antero Resources, which signed up to ship gas on the Rover pipeline, asked the FERC on Tuesday to decide "as soon as possible" so the project can meet its planned startup date this year.

"A delay would be a hardship on producers who have invested in anticipation of completion of the Rover project in 2017, as well as consumers who will pay more for their natural gas," the Denver-based company said.

National Fuel Gas. on Wednesday cited regulatory delays for pushing back the anticipated start date of the Northern Access pipeline expansion project to the second quarter of the company's 2018 fiscal year from November.

Other gas projects still waiting on permits include the PennEast shale line in the US. Northeast. On Monday, the commission delayed its review of the project, being built by a group of companies including UGI and Spectra Energy, by almost two months.

In October, the agency similarly pushed back a review of Williams Partners' $US3 billion Atlantic Sunrise gas project in Pennsylvania, sending the stock of one planned shipper, Cabot Oil & Gas, plunging. The project has since cleared its environmental study.

For their part, spokesmen for Williams, PennEast and Energy Transfer have said the companies still plan to bring their respective gas lines into service as scheduled.