Adds details and background

June 28 (Reuters) - U.S. pipeline operator Williams WMB.N is in advanced talks to acquire rival natural gas pipeline operator Momentum Midstream for about $5.5 billion, Bloomberg News reported on Sunday, citing people familiar with the matter.

The Tulsa, Oklahoma-based company is putting the finishing touches on an agreement to buy Momentum from private equity firm EnCap Flatrock Midstream, the report said, adding that a deal could be announced in about a week.

Reuters could not immediately verify the report. Williams Companies, Momentum Midstream and EnCap Flatrock Midstream did not immediately respond to a request for comment.

The deal would give Williams additional capacity to move gas from the Haynesville shale to U.S. Gulf Coast export terminals, the Bloomberg report said.

No final decision has been made and EnCap could still opt to retain the company, according to the report.

Williams is exploring acquiring U.S. natural gas production assets as it looks to secure supplies for its offerings to hyperscalers and data center clients, Reuters reported in February.

Momentum Midstream operates around 4,000 miles (6,437 km) of pipelines, serving more than 140 customers across its network, according to the company website. It also serves 10 liquefied natural gas facilities and 26 power plants.