THE European Community is looking increasingly to the United Nations as the world's best hope for resolving Yugoslavia's bitter civil war.But unless all factions agree to a meaningful cease-fire - 13 EC efforts so far have collapsed - any UN attempt to resolve the conflict with the help of peacekeeping troops may prove no more successful than past EC efforts. Also, it is by no means clear that the UN Security Council, despite its approval of an arms embargo against Yugoslavia in September, would agree to send peacekeeping troops or adopt an oil embargo against Yugoslavia as the EC recommends. China and some nonaligned members of the Security Council are wary of taking any action that might be interpreted as internal interference. The Soviet Union would need to poll its republics to develop an internal consensus on the issue. In response to a request made to UN Secretary-General Javier Perez de Cuellar by European members of the Security Council, UN special envoy Cyrus Vance is in Yugoslavia this week - his third trip in the last month - to assess the climate and requirements for what could become a sizable UN peacekeeping operation. Croatia, Serbia, and the Serb-led Yugoslav Army say they would accept UN peacekeepers but disagree on where the troops should be placed. The Serbs, who only recently embraced the idea, want UN forces stationed along the new front lines and around Serbian enclaves in Croatia. Croatia, which declared its independence last June but has lost almost one-third of its territory, wants UN troops deployed along traditional borders between the republics. Technically the UN is limited by its Charter to dealing with disputes among nations. However, growing acceptance of a global concern for human rights has allowed new inroads in some areas once considered each nation's own business. The Security Council did send peacekeepers into Iraq last spring to protect Kurdish and Shiite Muslim minorities. Also the UN will soon take on a wide range of activities - from peacekeeping and interim governing to organizing of elections - in Cambodia in the aftermath of its 21-year civil war. Janusz Bugajski, an Eastern European expert with the Center for Strategic and International Studies in Washington, says, however, that any UN peacekeeping involvement in Yugoslavia would be particularly dicey. "This is not a situation where you're simply keeping apart two states with clear borders," he says. "It's more of a guerrilla war where you don't quite know where the boundaries are. Those are extremely difficult to patrol, as we've seen in Northern Ireland and Lebanon.... It's not impossible to resolve this, but you need the goodwill of all sides." In addition to agreeing on where to place peacekeeping troops, UN members would have to decide who would supply them - Yugoslavia itself has long been a key supplier - and how they would be paid for. Mr. Perez de Cuellar has estimated the cost of a force in Yugoslavia at somewhere between $100 million and $200 million. The UN, which the secretary-general says has a current deficit of $1.2 billion, now runs nine peacekeeping missions at an annual cost of about $700 million. The Cambodian operation alone i s expected to cost $1 billion to $5 billion a year. Most current UN peacekeeping efforts have been launched within the last few years. Some experts question whether it is now too easy to turn automatically to the UN when the going gets tough. Edward Luck, president of the UN Association of the United States, says he hopes the UN can help in the Yugoslavian situation but that it is really more of a test for the EC than for the UN itself. The Charter obliges countries to try first to resolve their own disputes, he notes. Regional groups such as the EC are to help if that effort fails. The UN, which he describes as "virtually bankrupt" and facing a very full agenda even without Yugoslavia, was always intended as a last resort. Describing the EC as politically cohesive and relatively well off financially, Mr. Luck asks: "If the EC is going to admit failure, where around the world can we expect regional organizations to be effective?... The UN isn't going to be the world policeman any more than the US is." Still, Hurst Hannum, an associate professor of international law at the Fletcher School of Law and Diplomacy, agrees that "the UN can't resolve all the world's problems." But he adds: "This does demonstrate a newfound confidence ... that somehow it's legitimate for the UN to be discussing a civil war. I think it's probably a step forward even if members end up deciding they are constrained politically or legally from taking direct action."