President Trump said late Friday that the United States had struck a deal to take majority control of a huge portion of Venezuela’s oil reserves, pointing to a sweeping effort to assert U.S. dominance over energy in the Western Hemisphere.

The agreement amounts to a highly unusual foray into another country’s oil fields through a partnership with a private company, the State Department said through a U.S. official. The deal would give the United States control of more than 65 billion barrels of proven oil reserves, Mr. Trump said in a social media post. That is almost as much as all of the proven reserves in the United States, which is the world’s biggest oil producer.

Venezuela’s president, Delcy Rodríguez, called the agreement “historic” and said it would have a “significant impact on our nation’s revival.”

She said in a statement that the deal involved the development of 17 oil fields, more than $100 billion of investments and projected taxes of $209 billion for Venezuela’s strapped government, which has been struggling to improve living conditions after earthquakes in June left more than 6,500 people dead.

The aim of the deal, Ms. Rodríguez said, is “putting our immense reserves at the service of national development.”

Details were sparse, however, and the text of the agreement was not immediately available.

The Trump administration deposed and captured Ms. Rodríguez’s predecessor, Nicolás Maduro, in January, transforming a country that had been an adversary into something resembling a client state. As part of the process, the administration made Venezuela’s oil central to its campaign to expand U.S. influence over energy in the Americas, where rising production has helped blunt the loss of oil coming out of the Middle East over the past six months because of the Iran war.

Previous U.S. leaders often shied away from any suggestion that gaining sway over another country’s natural resources was a primary objective of U.S. military and foreign policy, wary of the international backlash, doubts over the legality of doing so and the potential for such moves to generate security threats against the United States.

But Mr. Trump and some of his top advisers have made it clear that oil is an important part of their foray into Venezuela.

Even before the removal of Mr. Maduro, American officials bluntly claimed that the United States had created Venezuela’s oil industry and that the country’s government had effectively stolen U.S. oil fields through nationalizations.

Unlike most other big oil powers, however, the United States does not have a national oil company through which it can make investments. Asked for comment, the State Department said through the U.S. official that the deal would in effect give the United States 55 percent of the output of a joint venture with an unidentified “experienced private operator in Venezuela.”

Oil exploration in Venezuela is generally conducted by the state-owned oil company, Petróleos de Venezuela, or through partnerships between it and private companies.

Mr. Trump made his announcement on Friday as Chevron, the second-largest U.S. oil company, was in advanced talks to significantly expand its operations in Venezuela, according to people familiar with the negotiations between the oil company and Venezuelan officials. That deal could be announced as early as next week.

Any new investment by Chevron, the only American company with a major presence in Venezuela, would bolster the country’s efforts to revitalize its ailing oil industry and its struggling economy.

Chevron’s negotiations over new investments in Venezuela were reported earlier by The Wall Street Journal.

Several other U.S. oil companies have also been pursuing opportunities in the South American country, though much of what has come to fruition so far this year has been preliminary or relatively small.

The Trump administration’s deal showcases the major shift underway in the energy landscape of Venezuela, which is thought to have the world’s largest oil reserves. Maintaining government control over oil has shaped the country’s politics for decades, and many Venezuelans view oil as a birthright and a cornerstone of national identity.

Under U.S. pressure just weeks after the capture of Mr. Maduro, Venezuela’s National Assembly approved an overhaul of legislation governing its oil industry, granting foreign oil companies greater control over their operations in the country. This move reversed much of Venezuela’s nationalization of oil projects in 2007 and opened the way for Chevron to expand its foothold.

The nationalization had been a pillar of Chavismo, the political movement created by Hugo Chávez, Mr. Maduro’s predecessor, which wielded control over Venezuela until this year.

Ms. Rodríguez, a Maduro ally who replaced him as president, has faced U.S. threats that she could meet a similar fate. She has been much more open to working with the U.S. government and frequently talks to and meets with top Trump administration officials.

Anatoly Kurmanaev and Lisa Friedman contributed reporting.