Washington ? Phillips Petroleum Co. and Conoco Inc. merged Friday to create the third-largest U.S. oil and gas company after receiving federal approval for the $15.1 billion deal.

The Federal Trade Commission voted 5-0 for the deal, but required the companies to sell refineries in Utah and Colorado and certain operations in Missouri, Illinois, New Mexico, Texas and Washington state.

Joe Simons, director of the FTC’s Bureau of Competition, said the agency believes the conditions attached to the approval will help maintain competition in the energy market.

The companies announced in November their intention to merge and to base the new company, ConocoPhillips, in Houston.

The deal creates the world’s sixth-largest oil and gas company. In the United States, ConocoPhillips is No. 3 behind Exxon Mobil Corp. and ChevronTexaco Corp. The companies said they completed the merger a few hours after receiving FTC approval.

The combined company is now the country’s top refiner and a gas retailing giant, with about 17,000 filling stations nationwide.

Fadel Gheit, an energy analyst with Fahnestock & Co., said the deal shouldn’t affect consumers or gas prices.

“It was a step that was necessary for both companies. They could not have survived single,” he said. “Bigger is better in the business where you don’t know where oil prices are going to be a year from now.”

In trading Friday, Conoco closed up 60 cents at $24.55. Phillips closed up $1.03 at $52.58.

The new company will have a market value of about $35 billion. Conoco shareholders will receive $15.1 billion in stock for their shares.

Conoco sells gasoline, diesel fuel, and other petroleum products at 5,000 outlets in the United States, while Phillips, based in Bartlesville, Okla., sells fuel at more than 12,000 stations under brands such as Phillips 66, Circle K, and 76.