WASHINGTON — The Pentagon has told the Obama administration that any military effort to seize Syria’s stockpiles of chemical weapons would require upward of 75,000 troops, amid increasing concern that the militant group Hezbollah has set up small training camps close to some of the chemical weapons depots, according to senior American officials.

The estimated size of the potential effort, provided to the White House by the military’s Central Command and Joint Staff, called into question whether the United States would have the resources to act quickly if it detected the movement of chemical weapons and forced President Obama, as he said in August, to “change my calculus” about inserting American forces into Syria. So far Mr. Obama has avoided direct intervention into the most brutal civil conflict to emerge from the Arab Spring uprisings, and the Pentagon assessment was seen as likely to reinforce that reluctance.

The White House on Thursday declined to comment on the Defense Department’s assessment.

The Pentagon has not yet been directed to draft detailed plans of how it could carry out such a mission, according to military officials. There are also contingency plans, officials say, for securing a more limited number of the Syrian chemical weapons depots, requiring fewer troops.

The discovery that Hezbollah has set up camps close to some of the depots, however, has renewed concern that as the chaos in Syria deepens, the country’s huge chemical weapons stockpiles could fall into the wrong hands. Hezbollah fighters have been training at “a limited number of these sites,” said one senior American official who has been briefed on the intelligence reports and spoke on the condition of anonymity. “But the fear these weapons could fall into the wrong hands is our greatest concern.”