BRUSSELS — The United States will deploy several hundred troops to guard eastern oil fields in Syria against the Islamic State, defense officials said Friday, another lurch in President Trump’s zigzagging military policy in the country.

Speaking at a news conference in Brussels, Defense Secretary Mark T. Esper said that the United States would “maintain a reduced presence in Syria to deny ISIS access to oil revenue.” Mr. Esper also said that the additional steps could include some “mechanized forces,” which other defense officials have said would include tanks that are not already there.

The plan includes a combination of Special Operations troops already in Syria and other units arriving from elsewhere in the Middle East, according to three defense officials who spoke on condition of anonymity because they were not authorized to speak publicly. The officials said the total number of American troops would be around 500.

While Mr. Esper denied the deployment represents another shift in strategy, it suddenly puts the number of American forces in Syria back on an upward trajectory — only weeks after Mr. Trump announced their withdrawal.