As Serbs batter, blockade and humiliate U.N. troops in Bosnia, NATO defense ministers called their top generals to an emergency meeting on plans to toughen the peacekeepers' defenses.

At the opening session of a two-day meeting Wednesday, the ministers renewed their commitment to keep the 24,000 U.N. peacekeepers in the war-torn country but pressed ahead with military contingency planning in case conditions worsen and a withdrawal becomes unavoidable."We have to prepare for a situation that nobody wants," said German Defense Minister Volker Ruehe. "We are attempting once again to make UNPROFOR (the U.N. Protection Force) more effective. But at the same time, we are in the preparatory phase for a pullout."

Senior U.S. officials said top military commanders from the United States and nine other NATO nations involved in the peacekeeping mission would meet Monday in the Dutch city of The Hague.

A senior U.S. official, insisting on anonymity, said the decision to increase the peacekeepers' defenses marked a dramatic turnaround at NATO.

Options for toughening the U.N. forces include redeploying peacekeepers to more secure positions; granting them increased powers to return fire if attacked; extending NATO air cover; and creating a fortified supply corridor from Sarajevo to coastal ports.

The peacekeepers, most of whom are lightly armed, also could be given more firepower. Officials noted forces from Denmark and other Nordic countries, backed by German-built Leopard tanks, have been more successful than others in deterring Serb attacks in northern Bosnia.

Counting their successes one truckload at a time, aid officials sent more relief convoys to Sarajevo Wednesday to bolster the Bosnian capital's badly depleted supplies.

Bosnian Serbs eased pressure on the United Nations long enough Tuesday to allow one convoy carrying 111 tons of food and fuel to reach Sarajevo. Another convoy reached Srebrenica, an eastern enclave also held by the Muslim-led government. Srebenica and Sarajevo are U.N.-designated "safe areas."

Peter Kessler, a spokesman for the U.N. High Commissioner for Refugees, said two convoys with 239 tons of food were sent to Sarajevo Wednesday. Serbs stopped them for inspection outside the city, but the trucks were expected to reach the airport by evening, said U.N. spokesman Kris Janowski.

Thirteen more convoys were planned for Muslim enclaves around Bosnia during the day, but it was not clear if the Serbs would let them through.

Gen. John Shalikashvili, chairman of the Joint Chiefs of Staff, will attend The Hague talks as will the defense chiefs from Italy, Belgium, Norway, Denmark, Spain, France, Britain, Turkey and Canada. U.N. commanders in Bosnia, and senior officers from non-NATO nations with troops on the ground were also invited.

"There is a common agreement that we want UNPROFOR to stay and a realization that we need to explore ways of making UNPROFOR more effective," said British Defense Secretary Malcolm Rifkind.

But NATO military strategists were completing contingency plans for an evacuation. Plans call for up to 45,000 NATO troops, backed by tanks and warplanes, to ride shotgun if the peacekeepers are forced to retreat under fire.

The defense ministers welcomed President Clinton's offer last week of as many as 25,000 soldiers to join an evacuation force. The United States has refused to send any ground troops to Bosnia, angering allied nations such as Britain and France, who have thousands of troops there.

NATO sources said Greece, Germany and Portugal also said they would be prepared to help in any withdrawal.

German lawmakers are likely to block any request for troops to be sent to a region where memories of World War II Nazi atrocities against the Serbs are still vivid.