Questioning whether it would be effective or in the nation's best interest, the Reagan administration indicated considerable reluctance yesterday to impose the trade embargo on Iran that the Senate voted Tuesday.

"We support the overall objective of the {Senate} amendment fully," said State Department spokeswoman Phyllis Oakley, "but we feel that we need to analyze its implications."

The 98-to-0 Senate vote on an amendment to the defense authorization bill came in response to reports that U.S. purchases of Iranian oil soared last summer, providing Iran with $700 million for its war effort.

One of the concerns is that such an embargo would erase all pretense of still being a "neutral" in the Iran-Iraq conflict, an administration official said, contending that to do so would undermine U.S. efforts at the United Nations to gain broad international support for an arms embargo against Iran.

"People are looking for effective ways to influence Iran's ability to carry on the war. That's the bottom line and right now it seems the U.N. is the best way," the official said.

"There is a general consensus we'd all like to do something, but the question is what," he added. "You don't want to do something basically ineffective and the Iranians sneer at you."

Oakley and White House spokesman Marlin Fitzwater confirmed reports that an interagency group has been studying what measures might be taken to curb U.S. imports from Iran, particularly oil.

The study started early this summer, but deep divisions persist between the State and Commerce departments, which are most opposed to a trade embargo, and the Defense Department, which is more favorable to the idea, according to congressional and administration sources.

But there are widely differing views within departments and agencies on what steps, if any, the administration should take. Oakley indicated some policymakers are pressing for a total trade embargo, while others favor barring only Iranian oil imports.

U.S. policymakers have been especially divided over whether a boycott of Iranian oil would be effective given the administration assessment that trade sanctions imposed by the White House on Libya in January 1986 have proved a failure.

"Whatever we do must {first} be effective and, secondly, to the maximum extent possible, we mobilize the broadest international support for the measures we take," Oakley said.

"We want to be sure that whatever we do is effective in achieving the objective we share with Congress, which is to bring about a change in Iranian behavior," she added.

Secretary of State George P. Shultz and other State Department officials have argued that an oil embargo is difficult to monitor because part of the world's crude ends up in an international "pool" and this makes Iranian oil "fungible."

In a recent interview, Assistant Secretary of State Richard W. Murphy said the United States had discovered it would be "extraordinary difficult" to impose an oil embargo on Iran. He said the trade embargo on Libya showed it was "extremely difficult" to trace the origin of an oil shipment once it entered the international market.

"If the price is right and Iran has heavily discounted its oil, Iranian oil will flow to other markets," Murphy said.

Another administration concern has been the effect of a trade embargo on the 1981 Algiers Accords, under which U.S. hostages held in Tehran were released. Oakley said the United States still considered itself bound by terms of the accords.

But the accords had only stipulated that the U.S. economic sanctions on Iran then in effect be lifted, she said, implying a future embargo was not legally excluded by the agreement.