U.N. Troops in Bosnia: Relief and More Grief : Balkans: Serbian rebels allow supplies to reach Bangladeshis. Then they fire a missile into Bihac.

In the one-step-forward, two-back pattern of the U.N. mission here, officials secured the release of 20 Canadian captives Thursday and piloted supplies through to embattled Bihac while Bosnian Serb rebels attacked with missiles and continued to hold hundreds of other peacekeepers hostage.

Shortly after a 14-truck convoy of food and fuel for 1,200 ill-supplied Bangladeshi troops was let through Croatian Serb roadblocks, Bosnian Serb fighters fired a surface-to-air missile into a civilian area near the main U.N. base in the Bihac pocket, mission spokesman Paul Risley reported.

Risley said the missile damage will be examined at first light today. While hailing the convoy’s arrival as a minor breakthrough, he condemned the SA-2 missile attack as “a weapon of terror.”

“An SA-2 fired toward a ground target is an extremely unstable, imprecise missile packed with high explosives,” Risley said.

Bosnian Serbs earlier in the day released 20 Canadian troops held for more than two weeks at a jail in the Sarajevo suburb of Ilijas, but nearly 300 other U.N. troops remained in detention, Risley said.

Thirty-five other Canadians confined to their observation posts near the town of Visoko were allowed to rotate out, but their newly arrived replacements are still virtual prisoners.

Gunmen loyal to Bosnian Serb warlord Radovan Karadzic took hundreds of U.N. troops hostage after North Atlantic Treaty Organization air strikes in late November as insurance that their forces would not be hit again. Some of the captive peacekeepers have been starved, mistreated and used as human shields.

The small letups in a campaign of harassment of the U.N. Protection Force that is increasing the likelihood of its withdrawal underscored the nationalist rebels’ dominant position. Repeatedly, they have demonstrated that the humanitarian and peacekeeping actions operate at their whim.

Although the supplies bound for the Bangladeshis were allowed through, Croatian Serbs detained an accompanying U.N. aid convoy in the town of Glina, just inside the Serb-held Krajina zone, Risley said. The aid was bound for the Bihac region’s mostly Muslim population.

The mission has been informed by the No. 2 in command of the Bosnian Serb army, Gen. Milan Gvero, that U.N. troops will remain captive and humanitarian works will be blocked until NATO warplanes abandon Bosnian skies.

“The Bosnian Serb military authorities are quite adamant that they are not going to give us the necessary safety and security guarantees for resumption of the humanitarian airlift into Sarajevo airport,” said the mission’s chief spokesman, Michael Williams. “They have said they are unwilling to do so while they are still under threat of NATO air power.”

Williams acknowledged the increasing appearance of rebel missile batteries around potential air strike targets had forced a reduction in NATO overflights, as the pilots have no authorization from the U.N. mission to destroy the antiaircraft missiles threatening them.

The air defense systems, which Western diplomats and NATO officials believe were provided by Serbs in the rump Yugoslavia, now provide cover over at least 40% of Bosnian territory, Williams said.