General Petraeus said that the acceleration of defensive systems — which began when President George W. Bush was in office — included “eight Patriot missile batteries, two in each of four countries.” Patriot missiles are capable of shooting down short-range offensive missiles.

He also described a first line of defense: He said the United States was now keeping Aegis cruisers on patrol in the Persian Gulf at all times. Those cruisers are equipped with advanced radar and antimissile systems designed to intercept medium-range missiles. Those systems would not be useful against Iran’s long-range missile, the Shahab 3, but intelligence agencies believe that it will be years before Iran can solve the problems of placing a nuclear warhead atop that missile.

Iran contends that it is not trying to develop nuclear weapons, and that its program is for energy production. The White House declined to comment on the deployments.

But administration officials who spoke on the condition of anonymity said the moves have several aims. “Our first goal is to deter the Iranians,” said one senior administration official. “A second is to reassure the Arab states, so they don’t feel they have to go nuclear themselves. But there is certainly an element of calming the Israelis as well.”

As Iran’s nuclear program proceeds — more slowly, American intelligence officials say, than the United States had once thought — Israel has hinted at various times that it might take military action against the country’s military facilities unless it is convinced that Mr. Obama and Western allies are succeeding in stopping the program.

Mr. Obama’s national security adviser, Gen. James L. Jones, took an unannounced trip to Israel this month, partly to take the temperature of the Israeli government and to review both economic and covert programs now under way against the Iranian program, according to officials familiar with the meeting.

American officials argue that the willingness of Arab states to take the American emplacements, which usually come with a small deployment of American soldiers to operate, maintain and protect the equipment, illustrates the region’s growing unease about Iran’s ambitions and abilities.