WASHINGTON (AP) _ Saying he worries that would-be terrorists could ''have a great country going into siege mentality,'' FBI Director William H. Webster on Thursday questioned whether it would be wise to close off a 90-acre area surrounding the White House.

''Sometimes excessive security actually invites trouble,'' Webster said when asked about reports that Secret Service officials are discussing putting a large area around the White House off-limits to vehicles.

Treasury Secretary James A. Baker III has acknowledged publicly that the Secret Service talked of a plan to augment concrete barricades placed in front of the White House in 1983 by closing a two-block stretch of Pennsylvania Avenue.

A more ambitious idea for a 90-acre vehicle-free zone hasn't gone beyond the talking stage yet, according to a White House official who discussed it only on condition that he remain anonymous.

''They have never formally done any proposal because I think the obstacles are so tremendous,'' the official said. Asked how serious the idea was, the White House official replied, ''From their (the Secret Service's) point of view, I think it's serious.''

In an interview with news service reporters, Webster noted ''the devastating effect'' of the suicide truck bombing of the U.S. Marine barracks in Beirut in October 1983 that killed 241 servicemen. He said he understood the concern of the Secret Service but added that he knew of no specific plan.

'' ... The poor guy in the security business. It's (security) always too much until there's a breakthrough,'' Webster said. ''And then it's not enough.''

'' ... Everyone associated with security is sensitive to how much damage can be done if some means are not taken to anticipate or make access more difficult,'' the FBI chief said.

But Webster added that ''I happen to favor what I call the prudent approach, which is to do what is prudent and to do it as inconspicuously as possible ... consistent with the risks involved.''

Huge concrete barriers were placed in front of the White House in 1983 to thwart potential suicide truck bombers. The belief is that a vehicle-free zone would not only prevent penetration of the White House compound but also detonation of any bomb.

Webster said excessive security steps can give a would-be terrorist ''a sense of having achieved success without having to run any risks.''

''If he can have a great country going into seige mentality, it is a form of victory for terrorist groups, and we shouldn't do that. We should take prudent steps,'' Webster added.

He said, however, that ''a number of things have happened'' in the past five years to give authorities reason for concern, including the bombing of the U.S. Capitol in November 1983, which caused an estimated $265,000 damage.

''I think those barricades out in front and around the White House, which require you to slow down, were quite prudent,'' Webster said.

''Maybe they weren't the most artful things,'' he added. '' ... Along the way, somebody will think of something even less conspicuous. But how could we watch a truckload of explosive be hurled against our building down there in Lebanon and not wonder about leaving our principle institutions unprotected in any way?''

Webster said that when he travels in Europe, ''I see massive troops and large barricades in front of major buildings and installations.''

''I think, in the main, we're behaving very rationally in this country,'' he said.

''Of course we went through a period of vaporous intelligence about different kinds of hit teams coming to the United States,'' Webster said. ''That continues. But I don't think it has the city in its grip at all.''