WASHINGTON (AP) _ Defense Secretary Dick Cheney said Friday the United States must react with caution to changes sweeping the Warsaw Pact, but the revolutionary developments ''might portend some changes'' for the NATO alliance if peaceful reforms continue.

''The opening of the (Berlin) wall, it's a major event, but it's important for us to think about what comes after that,'' Cheney said in an interview at the Pentagon, minutes after the East German government announced that its open-border policy would become permanent.

Cheney said the U.S. government must develop a long-term strategy to deal with the developments that have rocked Europeans on either side of the Iron Curtain.

''What kinds of governments will develop in the East, will they be democratically elected? ... What kind of security arrangements are going to exist between those nations and the Soviet Union? Do they constitute any kind of threat to the West?'' Cheney asked.

''If you get non-communist governments in Eastern Europe, they're likely to be less of a threat than has been true in the past in the military sense, and that obviously might portend some changes on our side,'' Cheney said.

The secretary said the United States and the governments of the North Atlantic Treaty Organization, ''need to be cautious.''

''We need to digest these changes. Clearly we welcome them, but we have to be little reserved about formulating major policy shifts until we have an opportunity to see what happens,'' Cheney said, adding that he finds the developments in Poland, Hungary and East Germany ''very positive. The trend's very much in the right direction.''

Cheney has said repeatedly that he doesn't see the need for any immediate changes in the force structure of NATO or realignment of the alliance because of the reforms instigated in the East.

East German Ambassador Gerhard Herder also urged caution Friday, speaking at the National Press Club.

''I don't want to hide before you that these developments are very dangerous. They could lead, if they get out of control, to bigger conflicts,'' Herder said.

Herder noted that East German armed forces are only a short distance away from American, British and French forces stationed in West Berlin. ''By any miscalculation, they could be involved. This could lead to a conflict,'' he said.

A former military commander of the NATO forces, retired Gen. Bernard Rogers, said Friday the changes in Europe will result in pressures to reduce the 340,000-member U.S. military force in Europe.

''We'll find that there will have to be withdrawals from Western Europe, because we can't afford to keep our troops there,'' Rogers said on NBC-TV.

Rogers, who served from 1979 to 1987 as NATO's commander, said a partial U.S. withdrawal would require the revision of NATO strategy, affecting the so- called ''forward defense'' that keeps U.S. troops on the front lines in Europe should war break out.

The retired general, asked as well about the possible reunification of Germany, said he believed ''the events so far lead us in that direction,'' but that the Soviet Union and many nations in Western Europe are ''very concerned'' about such a development.

Cheney, asked about U.S. support for Germany in the midst of the refugee influx, said he'd sent orders to Gen. John Galvin, NATO's European commander and the top U.S. military man in Europe, to provide ''whatever assistance he can'' to the West German government.

The defense secretary emphasized that the Soviet Union has said it wants preserve the Warsaw Pact, ''and I don't see there's any need for us to challenge that at this point.''

He said that while he, President Bush and Secretary of State James A. Baker III all welcome the changes, ''We want the changes to occur peaceably.''

''While we want to encourage freedom and democracy, it's the kind of change we hope will occur without violence, without a resort to force,'' he said.

Cheney said that at this point, it appears there have been no alerts called among the East European military forces, and that among the East German police, border patrol and military ''the spirit, the mood is very positive, very upbeat.''