“Until we have the ability to access more Canadian heavy oil through improved infrastructure, the vulnerability will remain,” said David L. Goldwyn, former State Department coordinator for international energy affairs in the Obama administration. “The potential for an obstruction of the Strait of Hormuz therefore poses a physical threat to U.S. supply as well as a potential price shock on a global level.”

Obama administration officials said they were not overly worried for several reasons. In the event of a crisis, the United States could always dip into strategic petroleum reserves; domestic production continues to climb; and Gulf of Mexico refineries could be adjusted to use higher-quality, sweeter crude oil imported from other countries.

“There are going to be tensions in the Middle East whether that oil is going to the United States or going to somewhere else,” said Adam Sieminski, administrator of the Energy Department’s Energy Information Administration. “And if oil prices go up because of a problem in the Middle East, that causes a problem for the world in general and not one that is specific to the United States.”

In the United States, several oil refining companies have found it necessary to buy more crude from Saudi Arabia and Kuwait to make up for declining production from Mexico and Venezuela, insufficient pipeline connections between the United States and Canadian oil sands fields, and the fallout from the 2010 BP disaster, which led to a yearlong drilling moratorium in the Gulf of Mexico.

“As refiners, we buy from wherever the supply is readily available and where we can get the best price,” said Bill Day, a spokesman for Valero Energy, the largest domestic refiner.

The United States imported a daily average of more than 1.45 million barrels of Saudi crude over the first five months of this year, compared to a daily average of roughly 1.15 million barrels over the same period last year, according to Energy Department estimates. Similar increases have come from Kuwait and Iraq, even while total OPEC and non-OPEC imports declined. The United States has imported little Iranian oil in recent years.

In recent years, United States oil imports have been trending lower, although total imports are little changed from the end of last year. But the big change came in imports from the Persian Gulf, spiking to 2.6 million barrels a day in May from 1.9 million barrels a day last December, now roughly 23 percent of total imports, compared with about 17 percent before.