IN an unprecedented move, the United Nations Security Council imposed a mandatory arms embargo on Yugoslavia Wednesday - with the full support of Yugoslavia's federal government.Yugoslav Foreign Minister Budimir Loncar asked the Council for its help: "Yugoslavia is in conflict with itself.... You know only too well that we have not been able to resolve the crisis on our own." He said that historical, political, and especially ethnic differences were involved. Open violence erupted in Yugoslavia this summer, after the republics of Croatia and Slovenia declared independence. The Yugoslav Army, in defiance of federal authorities, mounted land and air attacks against the breakaway regions. The Yugoslav foreign minister urged the Council to adopt "a general and complete embargo on all deliveries of weapons and military equipment to all parties in Yugoslavia." He said the action was needed to reinforce a fragile truce that has followed several broken cease-fire agreements and to encourage the opening of a peaceful political dialogue. Yugoslav journalists at the UN said that "at the moment, Yugoslavia is one of the best markets in the world for weaponry - and not just small arms, but Stinger antiaircraft missiles, rocket launchers, and antitank weapons." They also estimated that the Army had lost control of 30 to 40 percent of its fighting units. The journalists also said US Secretary of State James Baker III may have encouraged the intervention of the Yugoslav Army during a visit in July, when he said the US would not accept the Slovene and Croatian declarations of independence. But on Wednesday, Mr. Baker, who led the US delegation in the Council meeting, bluntly accused both the Yugoslav Army and the militant leadership of Serbia of "special and indeed growing responsibility" for the situation. "Clearly, the Yugoslav federal military is not serving as an impartial guarantor of a cease-fire in Croatia," Baker said. "On the contrary, it has actively supported local Serbian forces in violating the cease-fire, causing deaths to the citizens it is constitutionally supposed to protect.... It is equally clear that the Serbian leadership is actively supporting and encouraging the use of force in Croatia by Serb militants and the Yugoslav military. The apparent objective of the Serbian leadership and the Yugoslav military working in tandem is to create a 'small Yugoslavia' or 'greater Serbia.' " Baker touched on another sensitive issue when he told the Council that "this new entity would be based on the kind of repression which Serbian authorities have exercised in Kosovo for several years." The largely ethnic Albanian, Muslim population in Yugoslavia's restive southern province of Kosovo has chafed under Serbian-imposed martial law for years. This issue has become a matter of bitter reproach between Yugoslavia and Albania during the UN's annual general debate. Baker also expressed concern that the current fighting might spill over into Bosnia-Herzegovina. He said that the US was concerned about the "dangerous impact on Yugoslavia's neighbors, who face refugee flows, energy shortfalls, and the threat of a spillover in the fighting." The high-level Council meeting was chaired by the French foreign minister, Roland Dumas. In all, 11 foreign ministers participated, including those from Austria, China, Cuba, Ecuador, India, Romania, the Soviet Union, Britain, and Zimbabwe. They asked the UN secretary-general to get involved without delay, and to report back to the Council. The ministers voted unanimously to invoke the rarely-used mandatory provisions of Chapter 8 of the UN Charter. These powers were also used in response to Iraq's invasion of Kuwait. Several nonaligned delegations as well as China said they agreed to the resolution only after the Yugoslav government itself asked them to support it. Yemen's Ambassador Abdalla al-Ashtal expressed concern about the Council's "experimentation" in settling disputes. "We think it is important not to ignore the principles of the UN Charter.... And the Charter stipulates noninterference," he said.