Bosnian Serbs Press Offensive : Balkans: Thirty-two Dutch soldiers are taken hostage as tanks and infantry spearhead assault on ‘safe area.’ United Nations deploys rapid-reaction force.

Bosnian Serbs, in their most brazen attack on a Muslim enclave this year, advanced tanks and infantry on the U.N.-protected pocket of Srebrenica on Sunday, killing civilians and taking 32 U.N. peacekeepers hostage.

Pushing a 2-day-old offensive, the Serbs overran four U.N. observation posts and moved their tanks to within half a mile of the eastern Bosnian town, where more than 40,000 Muslims are harbored.

Seventeen Dutch peacekeepers were taken captive by the advancing Serbs on Sunday, adding to 15 seized the day before, U.N. military spokesman Lt. Col. Gary Coward said.

In a rare show of U.N. force, an elite rapid-reaction team armed with antitank missiles was deployed to stave off the Serbian attack, and the United Nations threatened to unleash air strikes by North Atlantic Treaty Organization jets.

The move on Srebrenica--one of six U.N.-protected “safe areas” in Bosnia-Herzegovina--appeared to be part of a steady pattern of escalating Serbian attacks in the last several days that included firing on the helicopter carrying Europe’s chief mediator and the shelling of U.N. headquarters in Sarajevo, the capital.

It posed yet another daunting test for the U.N. peacekeeping mission here, which has been repeatedly cowed by the Serbs--and ridiculed by the Muslim-led government--and whose overall presence in the former Yugoslav federation is seriously questioned.

Late Sunday, the United Nations dispatched an urgent warning to the commander of the Bosnian Serb army, Gen. Ratko Mladic, threatening to call in NATO air power if the Serbs attack the rapid-reaction force mobilized to protect Srebrenica.

“This attack against a U.N. safe area is totally unacceptable and represents a grave escalation of the conflict,” Coward said, quoting from the warning issued to the Serbs.

The developing showdown is the most serious confrontation between the United Nations and the Bosnian Serbs since NATO air strikes in May led to a disastrous episode of hostage-taking, the downing of a U.S. pilot and, ultimately, U.N. retreat.

“The Serbs have been pushing and pushing the limit,” one U.N. official said. “Maybe you can get away with a little shelling, but tanks rolling into Srebrenica is beyond the threshold.”

Civilians in the designated safe areas, which are predominantly Muslim, ostensibly fall under U.N. protection, although the United Nations has proved increasingly ineffective in protecting civilians or anyone else.

Coward said that an undetermined number of civilians have been killed in the Serbian onslaught of Srebrenica. A government official in the enclave told reporters in Sarajevo that eight people were killed and 27 wounded; other reports put the figure much higher.

In its warning to the Serbs, the United Nations demanded that the offensive be halted, that the peacekeepers and their equipment, including at least two armored personnel carriers, be released and that Serbian forces retreat to previously established confrontation lines.

Srebrenica was surrounded by the Serbs at the start of the war in 1992 and has suffered severe shortages of food and fuel. In an earlier attempt to take the city, Serbs attacked in April, 1993, prompting the United States to airdrop food to the starving area and the United Nations to order its protection as a safe area.

Since then, no such safe area has fallen into Serbian hands. But one of the eastern enclaves, Gorazde, came under furious Serbian assault in the spring of 1994, as did the northwestern enclave of Bihac in November, reportedly leaving hundreds of Muslims dead. Both offensives were met by NATO air power.

U.N. officials, struggling to salvage or redefine their role in the former Yugoslav federation, have privately debated sacrificing the eastern enclaves--Srebrenica, Gorazde and Zepa--which are difficult to protect and which leave U.N. peacekeepers vulnerable. But doing so would abandon tens of thousands of Muslims to the mercy of the Bosnian Serb gunmen.

“If Srebrenica falls, that will be the end of the U.N. mission in Bosnia-Herzegovina,” Mustafa Bijedic, the Bosnian ambassador to the United Nations, told reporters in New York.

Government officials Sunday implored the world do something to save Srebrenica. Bosnian President Alija Izetbegovic instructed his foreign minister, Muhamed Sacirbey, to ask for an emergency U.N. Security Council meeting, and Prime Minister Haris Silajdzic reportedly wrote NATO to ask for protection.

The Serbs intensified artillery attacks on Srebrenica early last week. Then, on Saturday, they overran a U.N. post staffed by Dutch peacekeepers. As the Dutch fled, one peacekeeper was killed, apparently by Bosnian government forces who may have mistaken the peacekeeper for a Serb. A hand grenade killed 25-year-old Raviv Renssen, who was operating a gun atop an armored personnel carrier. Izetbegovic has denied that his troops killed Renssen.

Three additional posts were taken Saturday and Sunday. Ten of the soldiers seized Sunday had first been prevented from leaving their post by Bosnian government forces.