Gazprom, the Russian energy monopoly, bought control of the world’s largest combined oil and natural gas development Thursday after a highly publicized campaign of pressure on its foreign operator, Royal Dutch Shell.

Shell’s sale of 50 percent plus one share followed months of accusations against the project by a Russian environmental regulator — a problem that President Vladimir V. Putin, in announcing Gazprom’s entry, said would now most likely be resolved.

Critics of the sale called it the first effective nationalization of a large foreign oil or gas project in Russia, which this year surpassed Saudi Arabia in oil production.

Mr. Putin announced the deal at a Kremlin meeting Thursday evening with executives from Gazprom, Shell and the Japanese trading houses Mitsui and Mitsubishi, which also own part of the project. He made a point of saying Russia remained open to energy investment.

“When speaking about the energy sector, we should admit this is a very liberalized sector of the economy,” Mr. Putin said. “All of the largest world companies are represented in Russia.”

Under the deal, Gazprom will pay $7.45 billion for the controlling share of Sakhalin 2, the vast energy project in Russia’s remote Far East, north of Japan. The project includes offshore platforms, 500 miles of oil and natural gas pipelines, a liquefied natural gas plant and an oil terminal.

The partners have so far sunk about $12 billion into Sakhalin 2, meaning they will recoup about half of their capital investment so far but will be compensated little for the estimated four billion barrels of recoverable reserves at the site.

The price Gazprom paid was “below market rate,” Alex Kormshchikov, an oil and gas analyst at UralSib, said by phone Thursday.

Analysts said the price valued Sakhalin 2 reserves at less than $4 a barrel of oil equivalent, a benchmark in valuing oil and gas deals, compared with an average of $4.90 a barrel at large Russian oil companies like Lukoil or Rosneft.

Still, Shell’s chief executive, Jeroen van der Veer, said he welcomed the stability that an agreement implied, after a turbulent few months when a Russian regulator threatened to halt work on the pipeline, claiming illegal logging and damage to salmon streams.

“I think the great news is that now there is stability so we can all work together, all the shareholders, to get the project up and running as soon as possible,” Mr. van der Veer said at the Kremlin meeting.

Shell reduced its share of Sakhalin 2 to 27.5 percent, from 55 percent; Mitsui to 12.5 percent, from 25 percent; and Mitsubishi to 10 percent, from 20 percent, according to a statement released by Gazprom. Gazprom and Shell also agreed to cooperate on unspecified future projects in Russia.

Russian authorities also backpedaled on their objection to a cost overrun announced by Shell in July 2005, agreeing to approve a doubling in the project cost now that Gazprom is a partner. The agreement was taken as another sign of the increased intertwining of government and business in Russia. About one-third of Russian’s oil and most of its natural gas production is under the control of state companies.

“Regulatory organs in Russia are essentially an arm of Gazprom,” Alex Turkeltaub, managing director of the Frontier Strategy Group, a risk consultancy in Los Angeles, said Thursday in a telephone interview. Mr. Turkeltaub, who has advised companies on political risk in Russia, said other Western oil companies operating multibillion-dollar operations here, like BP and Exxon Mobil, should now expect regulators to extract similar concessions.

At Sakhalin, Gazprom will honor existing contracts for delivery of liquefied natural gas after the project goes into production in 2008, but will control pricing and policies on all future sales, according to the statement.

On Thursday, Gazprom announced second-quarter profit of $5.2 billion, up 123 percent from last year.

The Sakhalin 2 sale came just two years after Russian tax authorities confiscated the largest production unit from Yukos and sold 76.6 percent at a rigged auction to a newly created shell company, Baikal Finance, for $9.4 billion. Yukos executives said the price was far less than true value. That pumping asset, now part of Rosneft, is valued by investors today at more than $60 billion.