An American official said on Wednesday that the Pentagon planned to present an option for deploying up to 20,000 troops to the region. But some Defense Department officials viewed such a move as potentially running counter to protecting the troops already there, as it could give adversaries in the region even more of a target.

“What we’re looking at is: Are there things that we can do to enhance force protection in the Middle East?” Mr. Shanahan said. “It may involve sending additional troops.”

Any additional troop buildup is bound to run into problems on Capitol Hill, where skeptical lawmakers — most of them Democrats — have challenged administration officials about whether the latest tensions with Iran have been caused by Tehran or Washington.

“I’ve looked at all the intelligence over the past two weeks,” Senator Angus King, independent of Maine, who caucuses with the Democrats, said Thursday on MSNBC’s “Andrea Mitchell Reports.” “The unanswered question is who is provoking whom. As we escalate sending more troops, moving aircraft carriers, we view it as preventative and as defensive. They view it as provocative and leading up to a pre-emptive attack.”

Top American officials, led by John R. Bolton, the national security adviser, and Gen. Kenneth F. McKenzie Jr., the head of the United States Central Command, have cited recent intelligence that they said showed that Iran was considering attacks by its proxy forces on American troops in the region and on the oil infrastructure belonging to American allies.