Defense Secretary Harold Brown, reflecting a hardening American demand for support from U.S. allies in the Iranian crisis, told West Europeans today that, while expressions of concern are appreciated, it was now time for "concrete economic and diplomatic steps."

Brown's public remarks, echoing a message that Secretary of State Cyrus R. Vance is privately conveying to the heads of several European governments this week, appeared part of a campaign to utilize all quarters of the Carter administration to exert influence on those governments and on public opinion overseas.

brown is in the Belgian capital to attend a crucial semiannual NATO meeting -- the alliance will decide tomorrow whether to deploy 572 new U.S. nuclear-tipped, medium-range missiles in Western Europe to balance a Soviet arms buildup of new missles and bombers.

The decision is the most critical and controversial to face the alliance in 30 years. It remains unclear if the Netherlands. Belgium and Denmark will go along with the new nuclear arms modernization plan at the special joint meeting of foreign and defense ministers here tomorrow.

Iran has become a dominating factor in the decision, U.S. officials traveling with Brown said, because lack of support for the new missle plan could be perceived as a crack in the image of allied unity that Washington is trying to project in its confrontation with Tehran. They said the Iranian situation illustrates the extensive U.S. commitments outside the North Atlantic area, and the special importance of NATO allies doing their share within the NATO region.

The officials said they felt Europeans did not understand the degree of outrage over Iran building up in the United States and the impression, whether accurate or not, that the Europeans either were not, doing enough to help in Iran or not, meeting their defense commitments.

At today's opening session, U.S. officials said Brown gave a stern speech to the allies, urging them to meet their defense spending commitments to NATO and perhaps even take on a bigger share.

The fact that the public warning in Europe came from the Pentagon chief rather than Vance, who normally is the spokeman on such matters, struck one top official at the meeting as both "astonishing and puzzling."

Brown's statement also could increase public speculation abroad that the United States may resort to military action if the allies do not go along with further economic and political actions.

At a brief and unusual press conference here, Brown was asked if he ruled out a naval blockade in connection with economic measures. Brown said he did not rule anything in or out, but that it is "very much preferable to pursue economic and diplomatic approaches, and we are making every effort to have those approaches be effective."

It was known that the U.S. officials were anxious to have Brown explain the U.S. view on Iran before an audience on an European television. This helps explain why he appeared before the news conference to read a statement and answer a few questions, even through the call for new steps in Iran was not discussed at this morning's NATO session.

In this statement, Brown said the action of the Iranian authorities in holding the hostages defied international law and was an assault not only on the U.S. but on civilized behavior and on all nations.

The denial of a request that neutral observers be allowed to see the hostages on a regular basis was a further outrage, he said.

"For these reasons, public support of all members of the alliance, indeed of all the civilized world, is undertandable in light of our common interest," Brown said.

He said "it is now appropriate, I think, for our allies and friends, indeed for the world community, to reflect its disapproval through concrete diplomatic and economic steps."

Brown would not say what specific measures the United States wants. When asked what response U.S. officials sounding out friendly governments have received so far, Brown said such talks are still going on, that the responses were, by and large, sympathetic but "it's clearly going to take additional discussion and additional work."