West German Chancellor Helmut Kohl today toned down his earlier endorsement of President Reagan's Strategic Defense Initiative, calling it a risk as well as an opportunity for the western alliance.

In a speech to NATO legislators in Stuttgart, Kohl said, "We cannot predict today whether SDI will prove to be an alternative means of preventing war and a way to reduce dependence on nuclear weapons."

"SDI means opportunity and risk for the North Atlantic Alliance at the same time," he said.

During President Reagan's visit to West Germany two weeks ago, Kohl expressed strong support for the SDI, or "Star Wars," research program and indicated that his government was favorably inclined to participate in the five-year, $26 billion project.

Kohl's ambivalence appears rooted in a desire not to distance West Germany from its European partners, particularly France, which has already rejected a role in SDI and is promoting a purely European project known as Eureka to accelerate space and high-technology research for civilian purposes.

The West German government has continued to advocate a joint European approach to the SDI program, but the different reactions in various capitals to the controversial space defense plan have made a unified European position increasingly unlikely.

The chancellor sought to deny that Bonn was now torn between allegiance to Paris or Washington, even though senior West German policy makers concede that a choice between SDI or Eureka may become necessary, possibly straining relations with the ally whose program is ultimately spurned.

"The often-cited dilemma of choosing between Paris and Washington does not exist for us," Kohl said. "My relationship with President Francois Mitterrand and our contacts with France are close and full of trust."

He contended that West Germany "accepts and supports" the French notion of pooling European resources and capabilities in a common research program to boost modern technologies.

But Kohl added that "the proposal to participate in SDI research remains on the table as well, and Europe will have to deal with this technological, political and strategic challenge from the U.S."

West Germany's final decision whether to join SDI research "will be guided by German, European and alliance interests" along with "careful consideration of the state of East-West relations," Kohl said.

The chancellor's hedged comments also seemed to reflect disparate opinions within his government. His senior advisers are known to favor participation in the U.S. program, while the Foreign and Defense ministries have raised occasional doubts.

Foreign Minister Hans-Dietrich Genscher has voiced support for backing Eureka and warned after seeing his U.S. and Soviet counterparts in Vienna last week that space defense plans were "incompatible" with detente and blocked all hope of progress at the Geneva arms talks.

Gen. Wolfgang Altenburg, chief of staff and the highest ranking officer in the West German Army, told the NATO legislators that SDI as now conceived raises more questions than answers.

Defense Ministry officials in Bonn say they are worried that SDI's enormous cost could divert U.S. resources from conventional defense, thus weakening the alliance's overall deterrence. West European economies could not afford to pick up the slack, the sources added.

In a gentle admonition to U.S. delegates attending the North Atlantic Assembly session, Kohl said any demand by Congress to withdraw troops from Europe if the allies do not increase their defense spending would be "inconsistent with American security interests and would send the wrong signals to the East."

Bonn plans to dispatch a group of government and industry specialists to Washington in the coming weeks to ascertain what kind of role in SDI could be played by West German firms. More than 30 companies have been identified by the West German Defense Ministry as possible contributors to the SDI program. Two optical companies, Zeiss and Leitz, are said to be world leaders in the field of optic sensors, while MBB is one of the top manufacturers of space subsystems.

Kohl also intends to meet Mitterrand in Bonn May 28 in an attempt to reconcile their views concerning the separate U.S. and European ventures in advanced space and technology research.

A senior policy maker in Bonn said the Kohl government is acutely concerned about the projected expense of the Eureka program since West Germany probably would be asked to assume the major burden. The appeal of SDI, he noted, is that so far "the Americans seem prepared to foot the entire bill."

In his speech today, Kohl insisted that the pursuit of SDI must not jeopardize the "coupling" of European with American security. He said that as long as no promising alternative is found, NATO must retain its doctrine of "flexible response," which calls for the use of nuclear weapons to defend Europe if conventional means fail.

But Kohl appeared to justify U.S. research in antimissile technologies by observing that Soviet Defense Minister Sergei Sokolov admitted recently that Moscow "has been conducting research in the field of strategic defense for quite some time."