Christopher Renews Threat of Attacks on Serbs : Diplomacy: Secretary issues warning at air base in Italy. NATO will deliberate Monday.

U.S. Secretary of State Warren Christopher warned Bosnian Serbs on Friday to end their siege of Sarajevo and abide by a framework peace accord that would give Bosnian Muslims a share in any partition of Bosnia or risk the threat of allied air strikes.

“The international community simply cannot accept the laying of siege of cities and the continuous bombardment of civilians,” Christopher told reporters assembled in a hangar just off the flight line at this eastern Italian NATO base.

“It’s time now for the Serbs to stop their strangulation of Sarajevo and the other cities of Bosnia,” he added. “They’d be well advised to take very seriously what we’re doing here. . . . We are ready to take the action that needs to be taken.”

Christopher made his remarks during a news conference after a tour of the Aviano base on his way back from a week of shuttle diplomacy in the Middle East. Aviano would be one of the allies’ major staging areas if the air strikes were ordered.

The visit was designed to bolster the credibility of the strike threat in the eyes of the Serbs and to underscore U.S. determination in advance of a NATO meeting in Brussels on Monday. At that meeting, the organization is scheduled to consider military plans for the allied action.

Aviano is one of about half a dozen bases in Italy and Turkey at which the NATO forces have assembled aircraft for use in a possible air strike. The United States also is making available warplanes from the aircraft carrier Theodore Roosevelt, which is on station in the Adriatic Sea.

Washington wants to use the threat of air strikes to prod the Serbs into ending their attacks on Sarajevo and hewing to the political framework agreed to in Geneva that would guarantee land for Bosnia’s Muslims after the country is partitioned.

U.S. officials fear that if Sarajevo, which is heavily Muslim, falls, or if the Muslims do not receive some land of their own, they eventually could become a radical guerrilla group that would jeopardize the stability of any peace accord signed without their blessing.

In its proposal to authorize air strikes against the Bosnian Serbs, the United States wants NATO to go beyond merely protecting U.N. forces in the country and preventing the fall of Sarajevo. It also wants to bombard Serbian-held supply depots and command centers in other sections of Bosnia. U.S. officials argue that such pressure is needed to prod the Serbs into giving land to the Muslims.

Under terms of an agreement worked out Thursday between NATO commanders and French Gen. Jean Cot, who commands U.N. forces in the former Yugoslav federation, both the United Nations and NATO will have to agree before any site in Bosnia-Herzegovina can be targeted for NATO attack. That agreement was approved by President Clinton’s top national security advisers Friday.

Cot and NATO commanders plan to meet today in the Croatian capital of Zagreb to work out a target list, Administration officials said. Failure to agree on targets could further delay the prospect of NATO air strikes against the Bosnian Serb forces.

Christopher received a briefing Friday from U.S. Gen. John M. Shalikashvili, who would head the NATO air operation, and U.S. Adm. Jeremy Michael Boorda, head of the U.S. Southern European command. Christopher also met privately with NATO Secretary General Manfred Woerner.

It was not immediately clear just how fully NATO will support the U.S. position on Monday. Britain, France and Canada, each of which has troops with the U.N. peacekeeping force in Bosnia, fear that the U.S.-backed air strikes might endanger their units.

Christopher expressed confidence Friday that the North Atlantic Council, NATO’s top political coordinating group, will approve the military plan sometime next week, asserting that the problems are “on the way to resolution.” He predicted that the session will see “a united alliance.”

But officials suggested privately that because of continuing political uncertainties, the council may be asked simply to approve the military preparations that have been made and leave the broader decision on whether to launch the strikes to NATO foreign ministers.

If that strategy prevailed, Christopher would return to Europe for the foreign ministers’ meeting, which would be held late next week. Some U.S. officials fear this strategy also could heighten the Serbs’ skepticism about the likelihood of allied action.

Earlier Friday, Christopher flew from Jerusalem to Damascus for a second round of talks with Syrian President Hafez Assad, carrying additional messages from Israeli Prime Minister Yitzhak Rabin in an effort to break the stalemate in the Israeli-Syrian portion of the Arab-Israeli talks.

Although neither side would give details of the messages, Christopher said they involved Israel’s answers to questions that Assad had asked Rabin, presumably concerning the terms of Israel’s offer to withdraw from the Golan Heights in exchange for a security pledge from Syria.

Times staff writer David Lauter, in Washington, contributed to this report.