HALIFAX -- A controversial plan to store natural gas in huge underground caverns north of Halifax is facing more delays and renewed scrutiny over its potential impact on one of Nova Scotia's most important rivers.

Alton Natural Gas LP, which had planned to start the process of creating the caverns before the end of the year, says that plan has been put on hold because of "project and regulatory planning."

And while the projected in-service date of 2022 remains unchanged, the company says it recently asked the Nova Scotia Utility and Review Board to extend its cavern construction permit to Sept. 1, 2023.

The company, a subsidiary of Calgary-based AltaGas, had initially planned to have construction completed between 2013 and 2018.

Alton Gas has confirmed work on the caverns can't start until the extension is granted.

Meanwhile, the province's opposition New Democrats issued a statement Monday saying Premier Stephen McNeil's Liberal government has failed to respond to questions about the project's compliance with federal regulations.

NDP environment critic Lenore Zann said she submitted questions to provincial Environment Minister Margaret Miller on Nov. 2, but there has been no response since then.

In the letter, Zann asks Miller if representatives from Environment Canada have told her department that part of Alton's plan does not meet federal regulations.

A spokesman for the provincial department said the province has not been told by the federal government that the Alton Gas project is not in compliance with its regulations.

"We expect this company to work with the federal government and ensure its project is in compliance with all federal and provincial regulations," Bruce Nunn said in an email, adding that the province also expects Alton "to consult and work with First Nations and the broader community as this project proceeds."

Earlier this month, a spokeswoman for the federal Environment Department, Veronica Petro, said the department can't speculate about the compliance of a facility not yet in operation.

Zann said she wasn't surprised to learn that federal officials have had little to say.

"In Ottawa, the ministers there are concerned about stepping on the toes of their cousins down here in Nova Scotia," she said. "We're trying to hold our minister's feet to the fire. We're saying, 'Come on, give us some answers. Grow up."'

As for Alton Gas, company spokeswoman Lori Maclean said Alton "must always be in compliance with all regulations and laws."

The project, on the drawing board for 12 years, has faced opposition from Indigenous protesters, who have set up a permanent protest camp near the Shubenacadie River.

The company wants to use water from the river to flush out underground salt deposits, creating caverns east of Alton, N.S.

The company says the leftover brine solution will be pumped into the river, twice a day at high tide, over a two- to three-year period.

Members of the Sipekne'katik First Nation in nearby Indian Brook, N.S., have argued that the project will hurt the 73-kilometre tidal river, which runs through the middle of Nova Scotia.

The river, which empties into the Bay of Fundy, is known for its unusually large tidal bore -- the wave created when the river changes direction as the rising tide in the bay pushes saltwater upstream.

Alton Gas has insisted the brining process will be safe, pointing to years of scientific studies.

"We are confident that as designed and operated, there will be no impact to fish or fish habitat from Alton," the company said in a recent statement.

"There have been in-depth discussions with the federal government, including Environment and Climate Change Canada, regarding Alton, and the project's engagement with government at all levels continues."

The storage caverns are needed by AltaGas subsidiary Heritage Gas, which wants to stockpile its product during the colder months to protect its customers from price shocks when demand spikes.

The company has said it plans to create two caverns about a kilometre underground. But the company has also said it may need as many as 15 caverns.

Each of the caverns are expected to be about 80 metres high by 50 metres wide.

The caverns would be linked to the nearby Maritimes and Northeast natural gas pipeline, about 60 kilometres north of Halifax.